You are quite lucky.
Last time my old employer did this, they changed everything as you have said, and then made everyone who was already employed, apply for one of the new positions!
If you you got your 'old' job back, then that was good. If you didn't, you had to find yourself a new position elsewhere. We even had to be interviewed for a job we were already doing anyway.
This was within a major high street bank so I'm quite sure that they must have ensured the legalities of it beforehand.
Luckily I got out and found myself a better job elsewhere first.