Hello Sam,
You should consider the open commitments when the data is transferred to FM and provide the budget for the account assignment objects in the go live year.
Now, if you say reassignment is done and the account assignment objects are proper for all the Pr/ Po's then reconstruct the documents. FMN4N if the number of documents is less and FMN4 if the number is large. You always have test run option to see if you get desired results or not.
Post that check for Availability control inconsistencies using FMAVCDIFF transaction, if there is any inconsistency then run FMAVCREINIT to correct them so that the budget values are proper.
I hope this clears your doubts
Regards,
Satan