First of all, even though I authored the book and put together that spell list, my opinion is unofficial. That is because it was, intentionally, unspecified. It's going to depend on how you want to handle it in your game/setting/whatever. So do what you feel is right for your game/setting.
The most important question, in my opinion, is: Is the actual specific weapon (i.e. that particular sword - or whatever) itself important to the character?
Is the Holy Weapon a simple tool that is, in the grand scheme, unimportant in and of itself? Meaning is the spell, the caster, and the deity 'allowing' the casting of that spell are what makes the weapon holy? If so, I'd say casting the spell on another weapon automatically cancels out the existing one. The power is in the faith, not the object.
This is really how I would use it since the character is going to likely find a better weapon at some point which it would want to make his/her holy weapon. But, if you want your deities to have an unfavorable outlook on losing ones bonded holy weapon without much effort to get it back then you might make the character jump through some hoops.