"7.1 Match Commitment
The listing of an applicant by a program on its certified rank order list or of a program by an applicant on the applicant's certified rank order list establishes a binding commitment to offer or to accept an appointment if a match results.
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Any program that offers a position to an applicant who matched to a concurrent year position in another program will be in material breach of this Agreement and the Match Participation Agreement Among Applicants, the NRMP, and Participating Programs and may be subject to penalties, as described in Section 9.0."
In order for you and someone else to switch programs, you have to get a hardship waiver from the NRMP to drop your appointment. Then, you can negotiate with another prelim program. If you negotiate with the other prelim program while still holding a match appointment, they are in violation of the match.