I have a slightly different take. Yes, you are contracturally obligated to the match result. But if you truly believe you will be unhappy at your matched program, you should speak to them like an adult, and accept the consequence of your decisions. It would be a miserable year for everyone involved if you don't bring your best. In theory, the right sequence would be to de-commit from your matched program, then reach out to the unfilled program of your interest, NOT THE OTHER WAY AROUND. I believe programs can see the applicant match results. Remember what they say about grass and fence.