It doesn't 'change' its address, it shows you a different one for convenience, that's all. As long as you keep wallet.dat you will have all the addresses it ever shows (and 100 more already made btw that you can't see).
I wish I had known that earlier. Perhaps other people will not have to make a thread like this one in the future.
Now that I know, and for the record - - →
I was not hacked.