Mass. Change to Electon Law for Kennedy Replacement is Violation of Constitution

Article I,. Section 10, in the first clause, the US Constitution says:

"No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility."

How can Massachuchetts pass a law after the fact of the the vacancy of Senator Kennedy without it being an ex post facto law????

I am emailing several Senators to ask this question, including the Minority Leaders and the leading Republican on the Senate Judiciary Committee.