Station staff whose roles are fully interchangeable and apply to swap leave are getting a big unreasonable No.
Say you are two former SAMFs, one covering down CSA1, the other covering down CSA2, then there is no reason why you could not swap leave.
No good reason, anyway. There is, however, the LUL reason aka. computer says no.
If this happens to you, don't accept it. Insist on your rights.