No, it can't be dynamically updated that way, although they CAN edit the final merged documents to correct anything they overlooked before the merge. But, they'll still have to remember to go back and change the data to match what they corrected. So, they've then doubled their work.
Often users request the impossible because they're too lazy to get their act together and enter/correct the data BEFORE they start the mail merge. Tell them they'll save themselves a lot of tedious work if they'll just do things in the correct order! ;)