according to implanon's manufacturer's website instructions on insertion, removal and replacing the implant, it says it can be replaced immediatley often through the same incision used to remove the previous implant. However it says that if any migration or excess tissue growth has occured (where they have to cut a bit more to get the old one out) then to prevent the same thing happening again (ie if you were to put a new implant into a deeper hole, it will take more to get it out next time) then a new site is usually recommended. The removal cut is about 2mm long.
Mand, South Wales, Full-time working, single mother to 2 scarily independent girls.
I AM A PROUD PLUMERIA SISTER