Chin implant placed Intra orally, and taken out submental approach - explanation?

It is easier to go back through the same incision. Your body will form a capsule around the implant (if silicone) or grow into the implant (if MedPor). This scarring starts just under the incision so it is easier to go the same way. It is not an issue going from the outside, sometimes advantageous to do a little neck lift at the same time because some volume will be lost when removing the implant. Just like anything, the surgeon's preference, background, and specific plan is the most important. Just make sure they explain the "why" so you know the thought process.

If your chin implant was placed intraorally then you should have it removed the same way in order to avoid a skin incision. If you had it placed from a submental approach, then you have already had a skin incision and as long as it healed fine remove it from the same extraoral incision. You didn't mention the type of implant (silicone or medpor) and that sometimes will affect my approach.