New characters was a certainly good idea to re-learn the way this game flows, but honestly it was probly unnecessary. Re-learning your old classes shouldnt be that hard. First take some time to read your ability and trait descriptions(pretend youre reading everything for the first time, because theyve done some serious rebalancing over the years and some things might not work like they used to anyway). If youre a melee look at how your combos fit together, if youre a ranged look at which abilities proc or boost which other ones. This is a good place to look for a general guide to see what the consensus is on how it all works. Then you can use level-synced content to re-fit everything together. Run a few synced fates(chocobo companion can help) in a level range, then maybe a boss fate or two in the same range, then a dungeon or two(the idea being that you increase complexity as you get comfortable - from spamming attacks at weak enemies to working over a single tough enemy to doing a group session where everything interconnects). At that point if youre comfortable with where youre at, increase to a higher level range(go up 5-10 levels) and do it again(again increasing complexity at your own pace). Somewhere in this process is where a more in depth guide can clear up any lingering misconceptions.

Of course, if youre one of those people who can just read a guide and have everything click without having to "do" any of it, then just do that. Regardless, welcome back and i hope you enjoy your stay!