Remember, votes are NOT for the president. Votes are for the ELECTORS. Who will vote as instructed. So if there is a swap (which there won’t be, and all this what if is pure alt-history nonsense) then the electors who were chosen to vote for the Democratic candidate will vote for the Democratic candidate.
Depending on state law, electors may be bound to vote for the specific candidate on the ballot, and their party may not have the power to direct or allow them to vote for a replacement. The Supreme Court has held faithless elector laws to be enforceable, and some call for the immediate, automatic disqualification and replacement of and invalidation of the vote of any elector who fails to vote as they are bound.