You were probably looking for the opinion of an expert on this, but in lieu of that you can have mine.

Pardons can't be reversed either by the president that granted the pardon or by a subsequent president.

My opinion is based on the idea that the constitution doesn't describe an undo pardon power and as such the president doesn't have one.

In addition, after Ford pardoned Nixon there was a lot of talk about ways to get around the pardon. All the powers that be at the time seemed resigned to the idea that the pardon couldn't be undone or gotten around in some way. If there had been a legal mechanism to get around the pardon I think it would have surfaced then.