This is one of Steve Schwartz's favourite questions so I'll answer it for him.

When you open the Control Accounts screen, do you see a message "Warning. Changes cannot be saved until you print and clear all posting journals. Do you want to continue?"?

If so, print & clear the Posting Journals first, then make the change.

Steve has pointed out several times that he would like to see this restriction removed on the grounds that it isn't necessary. However, until that happens, you must follow the rules!