Lol.. There's no way in hell I'm replaying quests at this point. I'm in it for the challenge of completing quests, not so much for the lore side of things. (I've never used any AP to this day). What I'm more interested in are the extra chapters and the additional area with Royal Edition. Do you know if it resets all the quests when you upgrade to RE???
Quests don't reset, unless you deliberately started a new game or used chapter select.
RE just added more quests. However, there are additional quests that are located in chapter 10, I think (I'm assuming you are at the end game frolicking in the world), if you don't mind seeing a few unfinished quests on your end game file, it's ok. Reddit says those quests aren't important.
As for the additional area in Insomnia, it can be played
(return to present via umbra)
but reddit says 1 boss (cerberus) will not appear if you've already cleared the game. Although some people said, everything is there including cerberus. I haven't touched the game since March and I haven't played RE yet, so i can't confirm or deny.
Also the royal boat can be used to move from lucis to altissia with some quests.
Either way, the extra save files can be used for things like lore accumulating by skipping chapters, or replaying chapter 14, if carberus doesn't appear in the end game file. I think I'd do this, if ever.