Being a dev myself, it very much depends on if they do serious changes to things like characters or switches that would be baked into your save file. A lot of the testing we do in our game can use the same save file as we progress through the game, but if its a dramatic change to things like, when you receive an item or if an event is changed you would have to start again in order to see those changes in your game without soft-locking your game. One thing that is possible, is making multiple saves as you progress through the game and you may be able to carry over your library of saves from one build to another. That said, I ofcourse understand that it is better entirely to start with a new save each time as a player you don't have developer knowledge of any small or large changes that happen between updates.
I don't know if this response helped you at all, but I hope it was atleast useful.