After 2029, the plane is to be transferred to the trump library, presumably refitted back to civilian spec, air force paying all costs.
There is no way in hell that the US would let Trump walk away with that gear.
10 years ago, we might have said much the same about the former President of the USA keeping most-secret classified documents unsecured in a bathroom of their resort.
Who knows what may actually happen.
Because it's not a small thing to change. You're basically overhauling everything if you wish to transition from a monarchy to a republic, because it's rooted in everything.
The names of the governmental positions, and possibly their responsibilities would need to change, as would official documentation, the money, the flag, the national anthem...
You could hardly call yourself a republic if your passports are still carry the authority of the monarch, and your national anthem prominently features the King.
It only gets more complicated if you're a former colonial power, since they may also be affected, and have to change everything as well. If the UK decides to ditch the Monarchy and become a Republic, Australia and Canada would need to follow suit, since it would be silly for them to have references to a monarch that no longer exists, or a GG who's meant to be representative for a position that no longer exists.
Either that, or there will be a political/legal headache deciding whether they become the new inheritors of the monarchy, since the parent is gone, or would they be also need to make the same changes (see above).