Deities can be rewritten in the DCU. Couple of different ways, in fact. Over time, the way a deity's followers perceive said deity can alter that deity to conform to that perception; deities also, as per Rucka, seem to have the ability to willfully reinvent themselves when provided sufficient catalyst and self-awareness. Frex, several of the Olympians, due to frequent exposure to Diana and her mortal, lateral thinking (note the ones who change most are the ones most involved with Diana over the years), were able to garner the self-perception to pull this off.
Urzkartaga has but a single worshipper remaining, and is probably also subject to Minerva's perception of him, which of course is even more limiting than Chuma's. He's never going to feel the slow-change, at least not in a direction that does him any good. But he also has a dead people, a fickle bride with no sense of obligation to or respect for him whatsoever, and a starvation diet, which is motive enough for catalysis; if you can *give* him self-awareness, it might be enough to allow him to deliberately reinvent himself as a more sapient deity. I'm not convinced anything Psycho could do could have that effect - we've never seen evidence he can touch or influence something that isn't even technically a mind - but it certainly seems within Circe's repertoire.
Once sapience is established, then, a renegotiation of the contract becomes utterly gratuitous - Psycho can simply steamroll Urzkartaga into doing whatever the hell Circe wants. If he can manipulate Hermes, a newborn god without a tenth of that power should be a cakewalk. This actually neatly accounts for the "what the hell happened with Donna and Angleo" problem - if the contract wasn't renegotiated, if Circe was just having Ballesteros do a half-assed placebo ceremony while holding Urzkartaga's leash through Psycho, then as soon as Psycho's concentration breaks, Urzkartaga rejects Ballesteros, finding the very idea of boy!Cheetah every bit as insulting and absurd as... well, as it is.
Urzkartaga then gives Cheetah her power back - to what extent he's able to and to what extent his influence is still affected by Circe's messing with Minerva back in Perez is an open question, but provides plausible justification for the sacrifice of Ballesteros' blood, the first such sign of fealty since way back when Minerva stole the lariat - but they soon enough find that both of them have changed too much for any prior arrangement to function, thus justifying *much* of the insanity between this and now. It still doesn't make Rucka's "doting wife" bullshit work, but some things don't deserve fanwanking...