Hello Eric
Please pay attention to the fact that there are two different types of locks.
1. There are locks of the material management and FI which can be unlocked in transaction MMRV, for example, "Allow posting to previous period". These locks will not allow to change the posting date and the error will go into COGI. Then you can change the posting date in COGI.
2. There is a CO lock for the confirmation, which can be set by using transaction OKP1 "Change period block". The period of "Activity: Actual Activity Allocation" needs to be locked there. By cancelling a confirmation of the locked period you will recieve message RU428 ("Posting period xxx yyyy is locked"). In this case, you will be allowed to change the posting date during the cancellation.
BR
Caetano