Depends on how the code works.
If the check is programmed as a loop- so as it will check which version this save will work with every time- it just might cause lags.
However, if the compatible version is checked once and then the properties are loaded, it may work much faster.
I guess you just say change is good. Even if you break saves too bad, just make new ones. It may break famous saves but those are old. Think of the endless possiblities with new elements to create and if you get a little sad every once in a while from your old saves just get an older version of TPT to look over them again. We do need change. We will never get anywhere without it despite the 'breaks' and the possibilities with the elements we've had since game creation are running out. Just think, almost every possible idea or creation has been made over then modified the made over then modified the made over them modified... Yes we could advance to inhuman advancements in TPT where everything is so connected and advance there is litteraly nothing to make... fine, but we need change to do what everybody knows they really want. New saves, ideas, challenges, and conversations for getting TPT to advance and fill up every possibility there is for new gameplay and interaction in saves and chats.
Its possible. We need to advance and the price is the price. I personaly think its pretty much the only way. Its kinda sad but how else besides only having new elements that can bend around it and not break to many saves. I and all of us would prefere that but we need to do what we need to do.
Like I said, we should still advance no matter the cost and we can still go back on past versions to see our hard work and remember it.
Yeah, what jacob1 said. Its not nececarily that we're going to break every and all of the best saves anyway.
I disagree with even minor breaks. Some people might not care about their work but I do, and I don't want to discover one day my best saves are "broken". "Breaking saves" is a perfectly legitament argument, and anyone who thinks otherwise thinks they can speak for everybody.
All we can do is leave it to the mods, and trust in their good sense.