The Flaw of "Improving the Sound"

I would not say that the new version changes its tone so much that it would obviously be noticeable. Changes have occurred more likely in the field of sound spatiality. In this I find more comfort for my ears (especially with prolonged practice in headphones). Perhaps, due to changes in spatial algorithms in the new version, you need to adjust the reverb settings. This can significantly change the tone. Or you have missed a setting that is different between the old version and the new version(speed curve, dynamics, settings of hammers ... yes anything ..) ..

Why not try a demo version first?

I have to disagree.

First of all, having dealt with multiple, rapidly evolving software and plugin sets in professional environments over the years, I consider the responsibility of maintaining expected functionality to be mine, as my own "system engineer." Security implications aside for the moment (that's a whole thing), there is rarely anything forcing adoption of new or updated software, and in fact you run the risk of "breaking" a smoothly operating system every single time you attempt it. When overseeing a render farm at a small mograph studio, we had to wait for calm weeks between major projects to first thoroughly research, check with vendors, then install and test new stuff to avoid interrupting ongoing projects with changes or workarounds. If you're in the midst of a series of recordings and need to maintain uniform sound without altering mixes or re-bouncing files, you best wait until you're done to change your toolset. Software is constantly in motion.

Secondly, in many cases the adoption of new software does not necessitate clearing the current or old one. Being an update rather than new version (i.e. pianoteq 7) this may not be as straightforward; unfortunately I don't have the previous installer handy to test. I suspect this was a tradeoff to avoid major version upgrade pricing in light of the relatively recent version 6 release, and personally I'm rather glad to have these additions for free.

Thirdly I thought the release notes describe the changes involved adequately enough. The "very unprofessional" move would have been to spring such alterations on users without indication, and though it is very rare, I have had that happen to increasing degree in this era of automatic updating (I'm looking at you, iOS)

I must admit I find this complaint surprising since, to my ears, 6.1 makes for a marked improvement in the subtlety, realism, and expressiveness of the sound and I like everything about it. It's actually put pianoteq into rotation where I'd used more specific sample libraries before. Improvement is by definition change...so neither expectation of sameness nor criticism of productive engineers makes much sense to me.

I could also argue that such an expectation would be similarly puzzling on a real instrument. I wouldn't expect to get the same recording from even the same piano and mic after a year for a variety of reasons, from tune to wear to performance. Virtual instruments have supplied us computer users a luxurious level of stability that is easy to take for granted. But in the end, if you simply must have an unaltered sound for indefinite future use, I suggest creating some actual recordings (bounce in place, should be simple enough) so you're not reliant on software actually regenerating the sound from MIDI events every time you need to change something....at which point I question the use of the term "finalized" anyway.

Or just freezing a particular track will do the same, pretty much.

It is interesting.
I understand the point of view wadecottingham in his need for mixing...
There are solutions as Joe and EvilDragon said.

Another solution could be to let all the old algorithms available in the last Pianoteq itself, with a menu to select 1 version.
This way, you could open Pianoteq and choose which version of the program you want to play.
The problem then is that everyone could play, record and post music made with an older version, and then give a bad idea of how it sound today... This could deserve the company if we assume the last is the best

I personaly find the 6.1.1 better than the old one, with just a little modification on the velocity curve to make it a little more sweet.

The demo version doesn't impinge on what you already run, ie Preq 5 or whatever.  When you download the upgrade, it encompasses your old presets whilst still goving you the flat velocity curve.
And if you revert to the old version, you'll find your preset defaults have got up and gone, for the most part.
Since little of pteq 6 is compatible to 5, you have to reset stuff which can drive an ole dude crazy, or to drink.  I prefer the latter.
But it's worth it, for you learn much on the journey.

Now, when 6.1.1 became installed on my computer, the settings seemed tailored to the computer and it plays without issue.  Sure, I'd prefer 1.3ms latency and more poly, but since I don't understand the implications of most of this stuff (ignorance is sometimes bliss) I tend to fiddle.

But I found the gem in this latest edition.  It is "Grotrian recording 3" which has the best ambience and overall grand piano sound I've ever heard recorded.  The upgrade was worth it just for that; a sound superior to that of almost any digital in production today.  Allegedly.
Best bit is I might be able to replicate the settings on D4 and K2 . . . .

Happy Days!