If a Ger candidate had a Mila, and before T'vela, decided against going through with the Geirus. If he ever changes his mind again and become a Ger, he wouldn't need Hatafas Dam Bris. Even though a circumcised Goy needs to have Hatafas Dam Bris in order to become a Ger (and here too he's coming to the second conversion already circumcised.) There, he wasn't circumcised in order to become a Ger. Therefore his circumcision is meaningless as far as Mila is concerned and needs a Hatafas Dam Bris. But here, where the original Mila was done to become a Jew, so the change of mind doesn't ruin the original Mila and they could precede by doing the T'vela.
.