If units are relatively the same size, it should work if you have a wait of 1 milisecond after removing the first unit. So, it is removed, there is a small wait, and then the new unit gets placed there. I used to have this problem in a defense. Because there was no time between the actions, it still considered the marine in that location. Try it with the wait, hopefully it'll save you from doing anything too long and complicated.
Um, if the unit is removed it's removed. Period. I know what you're talking about, but it's not because of the unit as it is a glitch with SC that occasionally pops up. Just use a center location action before creating the new unit (just center anywhere on anywhere... doesn't seem to matter apparantly) instead of 'wait 1 millisecond' or whatever.
By the way, any 'wait' or 'transmission' wait of 0 to ~80 milliseconds amounts to the same thing in SC. It has something to do with the time between "refresh" cycles in the engine, I think.
I did a test map to see and you were right (I attached it incase anyone wants to have a look). I used to have problems with this type of things, but I can't recreate it with this. Anyways, it won't be waits then. Gowith your scourge idea, or Tux's idea.