I agree with Road's. Actually, the rules expressly prohibit such a move but "logic" suggests perhaps it should be allowed. However, due to the "gamey" nature of the reason for doing that, then I think the letter of the rules should be obeyed:
7.3.3 MOVING INTO CITIES--DETACHING/ABSORBING FACTORS--GARRISONS: During a major power's Land Movement Step, any non-artillery, non-feudal or non-insurrection corps may detach factors as garrisons at, or absorb army factors from, depots and/or
unbesieged (emphasis added) friendly or vacant cities by reducing or increasing its strength, if the capacity is there.
The requirement that the city be "unbesieged" in order for a corps to detach/absorb would seem to be due to the inability to make the "move" through the besieging force. However, the rules don't carve out any exception for corps already insided a besieged city. Since the only reason for doing what is suggested is to avoid giving the opponent a political gain due to the absence of a corps (assuming no fortress), the "unbesieged" requirement for detachment should be strictly enforced, IMO.
