This is a known glitch. If you click "Continue," your new seat assignment is most likely locked in. The system is just failing to update the on-line seating information so you can't see it. If you call in to check, they will see you in the new seat even though you can't and your boarding passes will print for the new seats as well. This has happened to me on every trip since 3/3 on which I have tried to change seats.

Really annoying for those of us who are obsessive about checking seating assignments.