City support does not apply when disbanding units in a city to use the shields for a new unit. As long as the unit to be disbanded is in the city where you are building a new unit, its shields (well, one half) will be added to the unit under construction. The home city of the unit to be disbanded (it might even be a none unit), has no effect on the process. The only thing that matters is to be sure you set the city to building the new unit before disbanding the old one. Its a bad idea to disband the unit while still building, say, infrastructure, since when you switch to the unit, you will lose another one half of the shields....