If an allocation has been saved to the incorrect period and you with to allocate it to a period prior to this, you'll encounter the error Unable to allocate to an earlier period.
To allocate to a prior period you'll need to turn off the following option which is turned on by default:
Select System, System Control.
Select Options.
Select System Options.
On the Transaction Tab, un-check Allocate to the latest/year period of the transactions selected.
You will now be able to allocate the transaction to any open period.
This option helps cut down on user error, and we would recommend turning it back on when you've finished your corrections.