I presume that if Obama were to be elected that Electors pledged to vote for him would be freed to vote for another name on the Presidential ballot. McCain would probably be that choice, but it could theoretically be any of the others on the ballot in their state.
The Electors are not Constitutionally constrained to vote for any particular candidate. They can vote for anyone they want. Some states have Faithless Elector laws, but no one has ever been punished to my knowledge. There have been numerous occurrences of an Elector voting for someone other than the candidate to which they were pledged but it has never affected the outcome of an election. If 0 is disqualified prior to the EC vote then I would expect them to attempt to follow the perceived intent of the voters and switch to Biden as 0's legitimate successor, but they can vote for anyone at all. There won't be a redo as there is no Constitutional mechanism for it and there is a Constitutional remedy for an unqualified candidate. There is some flex in the Constitution that might allow a new election once all the explicit remedies have been tried, but that is the last resort and would require the SCOTUS to "reach" a bit, IMO.