I think many of us thought this day would never come. It's quite hard to think of something to say now it has.
I have not played a Final Fantasy game newer than VII. I've watched @kitation
play parts of each game until XIII, but I've not played them myself. I have no idea what to expect, but a wholesale replacement of the gameplay seems certain. The only "remake" I have ever played is Tomb Raider Anniversary, which I did not like, but the level design and gameplay are a much bigger part of the identity of Tomb Raider than for Final Fantasy VII. I stuck through the original VII gameplay even though I found it to be a chore, I can probably do it again.
I don't mind too much that Square Enix are updating the design and aesthetic of the game to bring it into line with contemporary RPGs. This is more-or-less what I expected. I'm only surprised that they did not take the opportunity to make this game in the last ten years, given the rapturous reception the PS3 Final Fantasy VII Tech Demo received. Having said that, a "remastering" approach would very much be welcome for mobile platforms.
What I'm more concerned about are changes to the game's story. Trends in the writing of remakes of well-loved franchises, such as Sherlock and Rebuild of Evangelion, have tended to go down the route of "remixing" story elements to provide something new an interesting for old fans. This has played out with mixed results across franchises. I don't trust the Square Enix of today to treat the story and characters with respect. I don't mean to say that I won't accept any changes at all, but it has to be done with respect.
I really can't wait to play it, especially now that the game's heritage and preservation is assured through PSN and Steam digital downloads.