It's a tight space in there. If you are thinking it's worth the cost of a battery, then you might open it up and look inside at what has to be undone to get to the battery. Some of those ribbon cables for the screen and other things that might have to be undone have very little excess length to let you open the case fully with out disconnecting them first. And they can be pretty fragile if you don't have the proper tools to pull and push on them. Or the fixture to hold the to case halves in the proper orientation while you use both hands for other things.
Certainly some or many have done it and in my opinion been lucky. But I also wonder how many did it and failed but are to ashamed to say.
I'd just get new. Some of the newer features are nice. Like knowing if someone in your contact list is out riding nearby and how far away they are.