The only way I made this work reliably was to take the media offline, rename the folders and then re-connect.
It's a pain, but it's not as painful as doing it one file at a time.
FCPX is so much better at this than Premiere Pro. I wish Adobe would get off the 'we play everything native' horse and start to understand there are times when editing natively is not the only game in town.
Sometimes, we want (or need) to edit with proxies and as we move steadily but surely towards 4K being the norm, frankly I don't want to edit in 4K native when proxy would be much easier all round.