I think the REM weapons will have some sort of trial/augment path to be brought in line with the new grade of weapons. Though I was reading FFXIAH and someone made the point that maybe they won't simply because they're just not that much of achievement anymore. If you have the gil, obtaining one is not particularly difficult and there's a lot of gil out there. Evidently fishbots in beaucadine can generate quite a lot of it lately and prior to the chocobo blinkers nerf you could turn a lot of cruor into a ton of gil with ease.
SE's already replied that R/M/E weapons will get big buffs to make them comparable to the new weapons. They also stated they don't really want to keep using the Magian Trials for them in the future. So who knows how it'll shake out. They gave a few examples such as:
(just listing the changes, and stats not finalized)
Excalibur- DMG: 73 Attack +40
to
Excalubur- DMG: 121 Attack +60 Accuracy +20