Then you'll kick yourself in the ass as you have to try to thread the halyard again. You'll worry about catching or setting up chafe of wires, twisting other halyards in the mast, or catching internal reinforcing in some masts.

Use Doug's (and others') advice on whipping. I think that you should make sure that the tape between the ends of the old-and-new" can make the turn over the block. It's principal use is to keep the end from catching something inside the mast or at the sheave as you feed it through.