There is no legal mechanism for unpardoning someone, the constitution only allows it to go one way. It would require an amendment, and we can't even pass laws with this legislature.
And for good reason, mind you. Same reason as dismissal without prejudice gets some flak as easily abused for corrupt leverage.
A future government is not necessarily bound to the one the preceded it depending on the way the one government changed into the other. And the way things are going right now there is a non-zero chance that the USA will see a reboot of its system of government, if it doesn't actually fall apart into multiple different countries.
And for good reason, mind you. Same reason as dismissal without prejudice gets some flak as easily abused for corrupt leverage.