Transfer carryover across assignments

Hi

How can we transfer carryover balance across assignments?
We have enabled Allow Prior Balance Reinstatement function, but it will transfer all the balance from one assignments to another one
However, some of the balance, is sometimes a carryover balance, with expire date, but when the balance gets transfered to the new assignment, both the carryover balance and other balance gets mixed, so the carryover have no longer expiration date

Our plan period ends 31.08, that remainin balance on 31.08 will expire on 01.01
When we have assignment shifts in these 4 months (01.09 - 31.12) for example, assignment 1 ends 20.10 the new assignment starts 21.10, the carryover balance gets transfered to the new assignment mixed with the new accural for September and Oktober, it will just be added on the new assignment as reinstatet balance so the Carryover balance will not expire on 01.01.