They switched actors for Daario Naharis, and that was in consecutive seasons. Benjen hasn't appeared since season 1. I have no idea what the actor from that season (not Sean Bean btw - he was Eddard Stark. Benjen was played by
Joseph Mawle) is up to these days, but it's conceivable to me that he could be easily switched out -- especially for a higher profile actor.
A glance at that page suggests Benjen will only appear in flashbacks. Don't know if that makes McShane more or less likely to portray him.