Legally almost impossible.
The convention system the GOP and Dems use really doesn't allow the types of political games and back door deals that occurred during the Civil War. Convention delegates chosen via primary are generally directly chosen by the winning candidate (i.e., they're Romney's guys). Caucus delegates, though elected at the local caucuses and thus not hand-picked associates of any particular campaign, are generally legally required to follow the state popular vote in at least the first round of National-level voting, or if their choice has endorsed another candidate, follow that endorsement (I may be wrong on at least this part).
Either way, for a sudden change in the candidate to take place, it would require not just some, but a majority of the delegation to violate their state laws or backstab the guy who hand-picked them for the convention, in what could only be described as one of the biggest political coups in recent American history.
The only way this would really occur would be if Romney himself bowed out of the election. And honestly... forget politics for a moment. Think of Romney as a person. He has literally spent about 6 consecutive years and millions of dollars running for President. He's finally being given the opportunity to run... and he's going to turn around and decline? Even if he was likely to lose, could you really imagine him turning away, and spending the rest of his life asking himself "what if I didn't decline that nomination" when the answer to that question is less than 3 months away?
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