Even if they have animation?
Are you talking about linking them into the same file they’ve been appended to?
You should be able to just delete them, link them back and then link the existing action from the dopesheet.
(Sorry, I didn’t see your reply earlier)
I will consider that next time. For now I already have animation on those characters, and I can’t dare to do anything that would slow me down. I will have to try with a new file just to learn using Link until I feel comfortable.
There’s no reason why that would slow you down, it’s really just a quick change, but your call !