To answer your question, we need to know: a) what stock/receiver extension do you currently have installed on the rifle; and 2) which MOE (rifle, carbine, or fixed carbine) stock you are about to have in hand?
Nope...no need for a buffer tube/receiver extension swap if its a standard AP4 as it shipped from the factory. The AP4s from the factory ship with commercial diameter buffer tubes and the AP4 collapsible stock so assuming you purchased the correct, corresponding MOE Carbine commercial diameter stock body, all you'll need to do is slide the AP4 stock body off and put the MOE on.