I think as long as there is not loot in the pool (so you couldn't need on everything) and you are not in combat you should be able to change in a dungeon regardless of whether or not the role is already filled. Sometimes, for instance, in 8 man content 2 tanks or heals are not needed, so ppl should be able to switch if they want to.

You could in v1 and in some instances it was almost mandatory. I think in AV you would down the first boss with melee and last with mages or something like that if you wanted speed run kills which were mandatory for darklight gear which was bis iirc. Yoshida gave us a reason about why it was changed but idr what that reason was. Regardless its just one of FFXIV's many many many needless restrictions.