This is a question that is asked quite a bit. The standard answer is No... you cannot change results once the UD is done.
SAP considers the UD to be a key event. Several things happen when a UD is made. If stock postings are required, the appropriate material movements are done. If batch management is activated, the batch values are updated. The LIS system is updated. Related originating documents like production orders/purchase orders/deliveries are updated. Notificatoins might be updated changed. DNR levels might be changed. To reverse a UD in order to change results requires a lot of checks.
SAP does supply a note that does allow you to undo a UD which then allows you to change results. SAP also provides many comments on this note and indicates that they do not support the changes suggest by the note. The client is totally responisble for testing, modifying and changing the any and all related code.