You can't make changes to your template's text (including fields) that will
have any effect on already created documents. That isn't how templates work.
You'll have to edit the individual documents. If your merge document is the
result document from a mailmerge and was saved, if you change the field to a
CreateDate field -- and update the field, it will give you the date that
document was originally saved.

Note, though, if you merely opened your merge document and scrolled to the
correct record or attached a database to it, you have not completed a merge.
However, if you did a SaveAs for that document when you did this, the
CreateDate field will give you the date of that save -- when you update the
field.