As for ethical ramifications, that is between the buyer and his/her own conscience.As for legal ramifications, that is a matter of contract law:1) Whether the buyer can change, when, why, etc. will be governed by the agreement between the buy and his or her agent.2) You write that the buyer made the offer and the seller has accepted it; therefore, the changing of the agent cannot alter the terms, conditions, dates, etc. of that contract. Even if it's awkward for the buyer to deal with the contract with a new agent, he or she must do so.However, as long as all contracts are complied with, the buyer should be able to do this.