soo every update need fresh start?
Nope. Just this one.
The game's internal structure got quite a big overhaul between ch1 and ch2. Hence the warning that saves might not work.
It seems to have been one of those "I can fix it now and affect 1 chapter... or fix it later and affect lots".
You could always copy your old save files manually...
On windows, copy files from:
C:\Users\{username}\AppData\Roaming\RenPy\Game-1545521833\
-to-
C:\Users\{username}\AppData\Roaming\RenPy\Thirsty for my Guest\
Copying the contents of the old save folder (.\RenPy\Game-1545521833) to the new one...
should work... most of the time.
But there are good reasons why some save files still won't work.
If one save file doesn't work... try another.
One aspect that will misbehave though is the sort of "fast forward through dialogue I've already seen". Practically EVERY line of code was changed, so RenPy will pretty much assume you haven't seen any of it before.
Honestly, some saves will work just fine...
But some won't. And it'll all feel a bit random.
As long as you're fine with that... just copy across your old save files and carry on...
if you can.
But to remove all doubt... start over.
It's only 1 chapter.