Even if they repealed the 22nd Amendment, we don't allow ex post facto laws, so the repeal wouldn't apply to him.
United States Constitution
Article 1, Section 9, Clause 3
"No Bill of Attainder or ex post facto Law shall be passed."
Article 1, Section 10
"No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility."
"an act of a legislature declaring a person, or a group of people, guilty of some crime, and providing for a punishment, often without a trial. As with attainder resulting from the normal judicial process, the effect of such a bill is to nullify the targeted person's civil rights, most notably the right to own property (and thus pass it on to heirs), the right to a title of nobility, and, in at least the original usage, the right to life itself."
Our repeated mistake is believing a guy that has no history of following rules and norms will suddenly decide to follow them as the most powerful "CEO" on the planet.
Changes to the law are only considered ex post facto laws in the United States when they bring about a criminal punishment - So prosecutors couldn't charge Trump if the 22nd amendment was changed to only allow 1 term, for example. So if the 22nd amendment was altered to allow for more terms, it would not be considered an Ex post facto law
Also within this very hypothetical scenario, the act of seeking a third term is after the hypothetical amendment, so there is no ex post facto in any case.
Ex post facto refers to criminal laws. Nothing to do with administrative processes. The retroactivity or not of such laws is based on a substantive due process analysis.