The point of the Colossus retrack isn't necessarily to improve the ride experience, it is to increase the lifespan of the ride. The location of the ride, couple with its age, has meant that replacing track is necessary really. If it has been to improve ride experience, I think there would have been higher priority sections than the start of the inline twists.
New trains may help, but I believe that the new style Intamin trains (including the ones found on the newer versions of the 10 invert coasters) are slightly different widths to Colossus', meaning it's not just a simple "buy new trains" job. Might not even be possible, unless Thorpe pay for a custom design, which increases the cost. That is, of course, on top of needing to retrack it.
Personally, I think Colossus has served its purpose for Thorpe. It put them on the map, a legitimate world-record breaking thrill ride at the start of their thrill-era. Now that they're established as a thrill park, it's time for it to make way. Part of me hoped that Exodus, being a standard sit down coaster with inversions, was being lined up as a spiritual replacement for Colossus. This retrack would suggest they're planning on keeping it for a good while yet though.