I think it all matters until the Electoral College meets in December to validate the results.
Can a concession be retracted?
Doesn’t need to be retracted. The final vote count is all that matters.
I’ve seen no evidence that a either a concession or a claim of victory has any legal basis whatsoever.
Each state will certify its votes and appoint its slate of electors in accordance with its election laws. Do any of those laws say anything about a candidate “conceding”?
If so, I would expect them to want a candidate to submit something in writing. A sound bite on TV would hardly seem to be sufficient. If a candidate officially withdrew, or declared himself to be ineligible, I suspect some states have laws that allow alternate electors to be appointed. But I haven’t looked up the particulars for any state.
In practice, every state will count their votes and then do whatever is easy and obvious when they certify. Candidates who don’t like that outcome can challenge in court (as in Floriduh in 2000). If “concede” is taken to mean “I don’t intend to sue you filthy dogs for trying to steal my votes”, then I guess a candidate can retract a “concession” in any state up to that state’s deadline for challenging the official result.