Of course I expect changes. But we aren't talking about changes in general. We are talking about a potentially super expansion of the game close to or beyond what's in current full AAA games that took years and millions to make and thats just episode 1 based on a tiny section of the original game
FFVII remake was announced in 2015, and they'd already been working on it prior to the announcement. It's now set to release in 2020 if there are no delays. That means it'd have been in development for over 5 years.
Considering that all or most of the artists from FFXV likely joined the team working on FFVII prior to FFXV shipping (basically when the game was finished and they were just focused on bug fixing for release), they've had plenty of time to make a LOT of content.
Consider this. FFXV was in development hell and the engine changed in 2012. Then the project was restarted in 2013 with a different story focus. They had to recreate a lot of the assets for the new engine and the higher spec machines (PS4/XBO versus PS3/X360). They still managed to ship a large open world game in 3 years after multiple reboots of the project. FFXV also likely had a smaller team than FFVII now has. After all outside of FFXIV, FFVII (all the various spin offs etc.) is Square Enix's biggest cash cow.
So, I don't think it's out of the realm of possibilities that they've created a fairly dense open world in Midgar. Of course, it also isn't out of the realm of possibilities that they've just padded the 2 BDs with pre-rendered FMV. However, IIRC, most of the cutscenes in FFXV were rendered in game, so I'd expect FFVII remake to be similar to that.
Also, keep in mind that RDR 2 was announced in 2016 (a year later than the FFVII announcement) and released in Oct. 2018, about 1.5 years before FFVII will release. RDR2 likely had more development time prior to the announcement than FFVII, but it's entirely possible that they both had roughly similar development times.
Regards,
SB