That's a really strange change, considering they didn't do any model updates for any other FF game on Steam. Not that I mind it here, but I'm really confused why they've clearly put so much more effort into FF9 than into fan-favorites like FF6 or FF7.
I bet they either used the battle models, or just had the slightly higher polycount models from the very beginning at the time of development but lowered them a bit for PlayStation rendering.
Field backgrounds however do seem the same, except they look worse, and even have buggy effects/animations at times (like the path to Memoria scene at the trailer at minute 1:01).
The FMVs are clearly the same just with a very nice filter pre-applied to them, and not original higher-res version (unlike what you could find for FF7 in the 1997 special DVD).
It is nice they're doing this, but... unless they improve the backgrounds with at least some well-thought method... it will still be better to play this on the original console or an accurate emulator.