The election can be made before the extended due date, if an extension was filed.  Section 6013.  And there is the special case of a surviving spouse, who may be allowed a year to change:

"Exception:  A personal representative may revoke an election to file a married filing joint return previously made by the surviving spouse alone. This is done by filing a separate return for the decedent within one year from the due date of the return of the surviving spouse (including any extension)."

I once used this provision to save a widow several thousand dollars after a Block-prepared joint return showed her owing for what would have been taxed only to her husband on a separate return.  Always think twice before preparing a joint return with a balance due, especially if one spouse is dead.  (We had to have her appointed as personal representative, but it was still worth it.)