Someone already posted a YT video of the differences in her singing (that didn't take long
), and I could immediately detect the slight difference in her singing. Certain inflections were a bit different, the effects at times were slightly different, etc. Plus, her voice sounds older and more mature now.
As for the legality of it, she is allowed to do it. I am pretty sure she wouldn't be if she wasn't allowed. I think the deal is she can release any re-recording of any of her songs from the Big Machine years five years after their original release, which is why we won't see anything re-recording from Reputation, for example, till late next year since that album was originally released in the latter part of 2017.
Additionally, because she still has veto power on the original recordings as far as commercial use, she can veto use of any of her songs from the first six albums in movies, TV shows, commercials, etc., that way the current owners of the recordings make no money from that venture. She will likely grant use of them from the re-recordings, of which she will completely own, so do not be surprised when songs of her start popping up in commercials, TV shows, etc., and any we hear will be Taylor's Version.