devs aren't going to introduce a way to backtrack the roster- they want you to keep growing the roster overall. its why they fix things that reward not investing in characters, like the undergeared nightsister team that was prevalent during HSTR years ago. its one of the big reasons they went to a skill rating system for GAC instead of just GP. downgrading can hurt TB's guild rewards for you and 49 other people.
if they changed their mind about this and did release a system, it would also make finding and reporting cheaters much harder since looking for journey guide character requirements is the only way some cheaters can be identified. there'd be a lot more false reports if something like this was added, diverting the dev's time and attention away from dealing with real cheaters.
as ultra said- there shouldn't be any regrets for relicing a toon.
upgrading by error? unfortunate yes, but that should be rare, if at all for the average player.
upgrading to find it not as strong as you'd like? this could be a mod/team issue or specific based on the teams it is being used against, but also solved by waiting to see what the whales and streamers have to say about it prior to upgrading.
an upgrading toon being good for a while and then becoming obselete? upgraded specifically for use with a DC or to be used in a particular raid team? you still benefited from it being upgraded during that time, so there should be no regrets there.
all in all, a system like this would be bad for the game as a whole.