I think there is a fundamental issue involved here: in accounting, there should be no changes once something is posted. There are many reasons for this, not the least of which are that the posted information could have already been used in financial information and that documents (like invoices) may have already gone out to customers so your system should reflect EXACTLY what went out.
There are a number of things we usually suggest if 'changes' are needed after the fact, depending on what transactions are involved:
1. Create an adjustment.
2. Void the original transaction and re-enter with new information.
3. Create a custom table to track the additional information you want and link it to the existing document number in GP.
All of these preserve the integrity of the system and provide an audit trail which is critical in an accounting application.