There was a strange sense of faltering change in the air in Lahti this September. Finland was at the end of its hottest and brightest summer for decades, but despite the shortening days the temperatures weren’t cooling and the sun wasn’t dimming. The Lahti Symphony Orchestra was revelling in its own new dawn, inaugurating principal conductor Okko Kamu with performances of the complete Sibelius symphonies. But at the drinks reception after Kamu’s fortifying Seventh, most of the talk was of an absent Osmo Vänskä and the famous recordings he made with this fine orchestra during his time on its podium.

Most of the talk, that is, but not quite all of it. When Robert von Bahr of BIS records interrupted the canapés to make a moving speech marking the completion of the label’s Complete Sibelius Edition (of which Vänskä was a linchpin), he added a deserved coda for Okko Kamu. His performances these last few days at the Sibelius Hall had been something special, von Bahr said, and the label would be releasing the symphonies under Kamu on disc. A complement, then, to Osmo Vänskä’s original Lahti set recently reissued as part of the Complete Edition. But before it will come another brand new Sibelius cycle: the second on BIS under Vänskä, who this time will conduct his orchestra in Minnesota.

By anyone’s standards, it’s an extraordinary level of investment in one man’s symphonies. But as Robert von Bahr spoke of the journey he and his staff had taken to finish the Complete Edition – a speech filmed on an iPhone by another of the project’s stalwart conductors, Neeme Järvi – it seemed that out of his exertions had come a renewed thirst. Von Bahr’s unerring belief in the music counters any accusations of overproduction directly: now is precisely the time for a fresh look at Sibelius – two fresh looks, in fact.

If anything could endorse that opinion, it was hearing Kamu’s Sibelius live in Lahti. I’d arrived with an iPod full of Vänskä conducting the symphonies with the same orchestra, the local band he made famous. But instead of forming a primer for the sort of interpretations on offer that weekend, they only provided a stark reminder of just how quickly performing styles can change, even for a single ensemble with such a strong tradition.

At the Sibelius Hall, Kamu’s Sibelius was hewn from entirely different interpretative rock, and the Sibelians from all over the world who make up the Lahti audience delighted in it. There was the residue of Vänskä’s intensity of atmosphere, but the overall feeling was one of going back to the notes on the page. ‘We don’t need to talk about the Sixth Symphony’ said Kamu to the interviewer who struggled to keep him engaged in the pre-concert talk, ‘we’re about to hear it.’

And we really did hear it, too – in all the glory of its vertical structure rather than the freewheeling, horizontal thrust we’re used to from Lahti. In the Sixth, Kamu highlighted the odd moment of transition as if to gently massage the sort of structural junctions that Vänskä would so brilliantly glide through (he changed down to a slower tempo at the pivotal modulation in the finale of the Fifth – a Vänskä bugbear). Harp, flutes and clarinets had the space to shine, maybe something of Kamu’s years at the helm of the opera house in Helsinki encouraging instruments to enjoy their moments on top of the orchestral texture and for a moment, perhaps, control and slacken the tempi themselves.

The Seventh had a weight of tone and tempo in those early string passages that conveyed the sort of vulnerability Vänskä always avoided. Put simply, there was a solemnity to the performance that shortened the breath.

The strange thing, then, is that listening to Kamu’s 1970 recording of the Second Symphony with the Berlin Philharmonic – the ‘prize’ session from his victory at the Karajan Competition – there’s an airborne thrust that feels like a pre-cursor to the Vänskä-Lahti sound. Conversely, Kamu’s 2011 Sibelius could only spring out of the sort of interpretations Vänskä made famous – in aesthetic, to begin with, but also in the secure technique and idiomatic familiarity of this ensemble which Vänskä almost single-handedly instilled.

The Lahti sound has certainly mellowed since Vänskä moved on in 2008. His immediate successor Jukka-Pekka Saraste (principal conductor 2008-11) probably initiated the deepening and broadening of the orchestra’s tone palette. Now, under Kamu, it plays with a feeling of warmth that helped make its Seventh far more affecting than the original BIS recording (the only account among the set that remains, for me, a misfire). ‘It was actually very regimented under Osmo’, reflects Lahti trombonist Vesa Lehtinen, responsible for that silhouetted solo in the Seventh. ‘In a sense he taught us the score. Now there’s more of a feeling of the score progressing logically – in a free way. With Osmo we had an approach; we knew exactly the way everything was going to be played. It was almost like a rock band atmosphere – and it was a lot of fun.’

It sounded so, too. There was always a thrill hearing this orchestra, live (twice at the Proms for many of us) and on disc. Many of those excitable young things recruited in the Vänskä regime are approaching middle age, and there was a certain wise unorthodoxy in the decision to appoint seniority and experience over youth and exuberance in Okko Kamu, 65 this year. There’s still urgency and drive, but there’s a sort of vintage patience and circumspection, too. It gets into those big acoustic chambers in the Sibelius Hall and helps create resounding panoramas aplenty that you hardly had time for amid all the wicked adrenaline of Vänskä.

But one thing certainly hasn’t changed: hearing Sibelius in Lahti is still unique. Among the many reasons for that, two stand out. Firstly that the musicians know so well how to use this indescribable acoustic, one of the best in the world for medium-scale orchestral music; and secondly the openness to interpretative nuance among this most dedicated of audiences, which comes from all over to hear an orchestra which has, almost like none other, such a tangible attachment to the composer it champions every September.

We might talk here of new sounds coming from Lahti post-Vänskä, but it should always be in the context of what an extraordinary character and advocate for orchestral music that conductor is – and what a brilliant, generous and visionary musician was created in him as he stood for two decades with the Lahti players in front of him.

And fifteen years after those legendary recordings were struck in Alvar Aalto’s Church of the Cross in Lahti (long before the Sibelius Hall was built), Vänskä is about to stride back in to the debate, big style. The first disc from Minnesota will arrive on mainland European shelves in March, the Second and Fifth Symphonies. According to BIS producer Robert Suff, the conductor’s trademark transparency and precision of articulation are preserved in the new recordings, but in the context of ‘a greater expressive range and a willingness to let the music breathe’. ‘I’m very proud of what we achieved together in Lahti’, says Suff, ‘but there is no denying the huge advances that have been made in recording technology over the last 15 years.’

If there was one overriding feeling in Lahti this September, it was of the orchestra and audience that Vänskä built similarly revelling in the magic of interpretative development and revisionism – the one thing that keeps great symphonic journeys like those of Sibelius and Mahler alive, and, indeed, keeps people reading the likes of Gramophone. There’s already been a thread on this website’s forum highlighting Kamu’s work in the sleepy Finnish town hemmed in by a lake and a mountain, and the fact that most concerts at its spectacular waterside venue can be watched on the web-streaming channel classiclive.com.

In the thread you sense something of the anticipation that a new conductor with this most venerable of orchestras in its ‘home’ repertoire can create – not to mention the anticipation of Vänskä’s new American Sibelius. For many of us, there remains something untouchably superhuman and simulating about this particular composer. What’s the only thing more exciting than a new Sibelius symphony cycle arriving on CD? Why, two new Sibelius symphony cycles arriving on CD, of course.

Watch excerpts of Okko Kamu conducting the Lahti Symphony Orchestra in Sibelius

Kamu and the Lahti Symphony visit Symphony Hall, Birmingham on January 26 and Cadogan Hall, London on January 27