you technically do not need to remove the old fets. removing them just looks neater. fet removing is the most frustrating thing ever, so i prefer just to stack on them if i have a 2x1 kit. if i have a 2x2 kit, then it's a different story...makes it easier.
but anyways, no you do not have to remove them.
it looks nicer, but it does no harm leaving the old fets there. (unles you completely fry a big stack, then you should remove them to keep an excess of bulk from building up)
__________________
|