Actor credited for one episode instead of full series

A particular actor was in every episode of a series, but he is listed as only being in one episode. I've tried to add him at the series level but it rejects, saying that he's already credited for one episode. Should I delete him from that one episode and then add him back at the series level? Or can I update his listing somehow to reflect his actual episode count?
(This is actually an issue with a few of the actors in the series, but I'm working on the guy listed on one episode before the one listed on 40!)
Thank you for any help.