Unposted MPA invoice journals cannot be updated

We have unposted Multi Period Accounting invoice journals in GL with source payables, in future enterable periods. These are created by the Complete MPA process. On the account combination is a department that used to be valid when the AP invoice was accounted, but not anymore. We would like to update the department segment on the journal lines, but this is not possible. Unchecking the 'freeze journals' checkbox on the Payables journal source makes no difference. So we can't post these journals.

Why are unposted MPA invoice journals never updatable? I don't see the point.

EBS Release 12.1.3

thanks

Abe