That would not be legal and would immediately be contested in court. The voter has a right to know how that vote will be counted before the vote is cast. If it turns out that these states, after the fact, don't like the results of the election and try to revert to the old way for just that election, there is nothing they can do about it until the next election because that would be changing the rules during the election.
Of course, that wouldn't stop them from trying...
-PJ
Under this compact arrangement, the voter does not know how that vote will be counted. The Electoral College vote in Massachusetts will not be based on the popular vote in Massachusetts. It will be based on the number of dead people who vote in Chicago. The current system has a clear result for a counted vote, so the voter knows.
Imagine a future election where the Democrat wins Massachusetts with 80%, but the Republican wins a squeaker with 50.1% of the national vote. You mean to tell me that the Supreme Court of Massachusetts is not going to figure out a way to let the Legislature cast those votes for the Donk favored by 80% of their population?
Eventually the matter would be settled in court, but there would be a heck of a lot of pushing and shoving before that happened.
Here is another nightmare scenario for you... Currently the Electors do not have to vote for the candidate they are "supposed" to vote for. In 2000, at least one Elector was threatening to change his vote, but it would not have made a difference, so nobody paid much attention. But how are you going to find an elector in Massachusetts who is going to go against "The Will of the People"?