Perhaps they could make a craftable item, maybe like a "Potion of Spear Based Forgetfulness" as an example to reset your Lancer level to 0 (with other classes having their own names for the potion, or maybe make it a book called "Moving on from Lancer"?). However instead of having to do the quests again, you just talk to the guildmaster and ask for a "refresher course" and they'll bring you back up to speed (either start at the level you start as for that class/job, or maybe bring you back up to the level you had before?). All abilities you had unlocked through quests will be unlocked by reaching the level for that quest. Good way to inject a niche item into crafters too.