MarkB
Legend
The patron collects its price from the warlock regardless, and as the warlock's power increases, the patron automatically gets even more from it. Verenestra gets to take more and more of your beauty as you grow in power, whether you have a continuing personal relationship with her or not. There's no getting out of that.
That's one way to play it. Another would be that it's an ongoing deal, reaffirmed every time power is exchanged. Each time the character beseeches Verenestra, he bargains away a little more of his beauty in return for more of her favour.
That's an interesting thought, but shouldn't the warlock have some say in it? I imagine that each archdemon's pact has very different benefits and drawbacks, according to its inner nature. It would really suck for the warlock if his "contract" could just be handed off to someone else and the nature of his powers totally changes.
Similar beings would have interest in similar souls, so there might not be too great a change. And for the rest, it could simply be one more pitfall of making deals with such dark entities.
This sort of thing would be mostly a role-playing choice, something to spice up a character's storyline. The character might have no say in it, but the player (in co-operation with the DM) would.