The actual MC transaction does not hold the period on it.  You just need to call up the GL transaction created when you saved the MC transaction and change the period to 6.  Go to GL Manager and set your Transaction Filter for unposted, period 5, source 1.  You should then be able to locate the GL transaction created by the MC transaction.  Call up the transaction, change the period to 6, and click SAVE.

NOTE – if you have already posted the GL transaction, you will have to correct the system by entering adjusting journal entries.