At its base I have absolutely no problem with the idea of attempting to ease him into a new nickname. As long as it still was short for Richard, I wouldn't mind at all. If it took it took if it didn't it didn't. Thing is, they comically blew ANY chance it had though (and even I admit the chance of it sticking otherwise still would have been small) completely and utterly by making it attached to this dumb gritty amnesia storyline.
The way to try it on for size would have been to do it with as little fanfare whatsoever within the narrative, imo.