I usually simply put another move on another button, then go back where the default attack is, and say to put that same move instead. It will swap them from one button to another.
So if you want to put pierce in slot 1 and light thrust out of the 1st bar onto the 3rd, just pick someplace on the 3rd bar, drop pierce there, then put pierce in slot 1. It will exchange them.
FFXIV: Cloe Delisle
Scholar, officer of the SWAGGER Free company, Sargatanas server.