Hi we had same situation. When we bought new tv sales person at best buy had us buy a certain mount as they one we already had was very old, wasn't sure it would work. Said the instillation team could tel us that.
They did, we were able to use the one already mounted for new tv, so we just took the newly one purchased back, but they would have taken down the old and replaced with the new one if necessary, and my understanding is that is part of the installation, no extra $. hope that helps.