I don't know if Eccleston will change his mind or not, but Tom Baker refused to do the 20th anniversary, then later filmed a scene for the 30th, and he refused to do audio dramas from 1999 on, then started doing them in 2012. People change their minds.
Didn't he say later, he regretted not appearing in the "The Five Doctor's"?
As for he 30th wasn't that a charity spoof? Charity can get people back into a role.