You're right except that electors in compact states go to the candidate who wins the national popular vote, not just the popular vote in the compact states.As I understand it:
With the NPVIC, as soon as there are enough states in the compact to represent 270 EC votes, then all the electors in those states are assigned to the candidate with the majority of votes throughout the compact states.
Therefore, any state that would, otherwise have electors going to the "losing" candidate on the national ballot, those electors would be assigned to the candidate with the largest plurality.
National Popular Vote Interstate Compact - Wikipedia
I agree about electors being forced to change their vote but that's the whole point. Electors would only be forced to change their votes from the national loser to the national winner. A state would only be losing their right to vote for the losing candidate. Is that so important to a state? That they have the right to be different? This argument of electors being forced to change their votes would only amount to a legal excuse for the courts to strike down the NPVIC. But the NPVIC is predicated on the assumption that the EC is obsolete so the courts would only be enforcing a law that most Americans do not want (assuming enough states join the NPVIC to activate it).