If the company went through the approval chain and got everyone to sign off on you being worth $comp + $relo, and you're value isn't effected by not choosing to $relo (because, in this case, you're already there), it should be pretty trivial to get everyone to approve a change to call that $comp + $signing_bonus since everyone involved already agreed to spend that portion of their budget on you. The case this doesn't apply is if their assessment of your value changes because they no longer trust you as much -- which does hint at an underlying rule of "even during a negotiation, don't be a dick" -- but the failure risk here is that you're providing symmetric access to the information that you're an asshole they might not want to work with, not that you asked for the compensation to be categorized differently.

So, yeah, I definitely have never solicited a bonus buyout and then done a bait-and-switch. But I've never actually solicited a bonus buyout -- I negotiate in terms of compensation I receive, not labels on it. I've had companies offer me bonus buyouts before, sometimes quite significant, and I've always taken that as a direct indication of their assessment of my value, not as something specific to the time of year.