I think some minor design changes for the suits wouldn't be very problematic, mostly changing materials, fixing proportions and adding details, like the The First Rider 1 and the first Rider 1 suit from the tv show. That was basically what Gundam did with the original design throughout the years, even for the giant statue. The colors all follow the original, but it has added details and tweaked proportions. I don't think anyone would complain if the old Riders were redesigned to that level.
However, even The First movie itself which got Rider 1's original outfit mostly right went and changed Rider 2's color scheme for no reason (giving him green stripes rather than white/silver and also making the green more grayish).
If they ever get around to making redesigns to replace the suits used by the Showa Riders in returning appearances, they'll need to keep the colors and general shapes, otherwise they just won't be accepted.