Running my macro as part of the mailmerge would require no more than:
Code:
Sub MailMergeToDoc()
Application.ScreenUpdating = False
ActiveDocument.Mailmerge.Execute
ActiveDocument.Range.Find.Execute FindText:="[0-9]{12}([0-9]{4})", ReplaceWith:="\1", MatchWildcards:=True, Replace:=wdReplaceAll
Application.ScreenUpdating = True
End Sub
Execution would then be via Mailings>Finish & Merge>Edit Individual Documents, the same as with any regular merge. Regardless, a macro really isn't needed...