. . . what a good question. I guess the answer is, it depends: A
subpoena comes from a grand jury, but usually the prosecutor handling an
investigation gets the subpoenas in blank and fills them out, both in
terms of what the subpoena duces tecum (here) asks for and in terms of
when the response is required, e.g., when the items are to be produced.
The grand jury is normally to be kept informed, but that is usually in a
very general fashion; they do not have to, for example, approve
subpoenas. So, if the FBI agent alters the subpoena I suppose that is a
ministerial act that would be considered acceptable if the prosecutor
(and perhaps the grand jury) were informed of the change and, indeed, if
the change came from them. If you're asking is that is something that
could be brought to a court's attention, I think it could. I also think
the recipient of a subpoena duces tecum should get a lawyer to advise
the recipient about the process of complying/moving to quash or modify
and about any issues such as these.