Now, clearly, this section did not contemplate the kind of allegiance-shift that Weaver did, becoming a de facto Democrat, while remaining nominally a Republican or an independent.  You could say that the Democrats all along had to nominate someone and that Weaver's resignation had no effect on that.

But, the law provides two ways for an incumbent justice to be nominated -- by affidavit or by political party.  So MCL 168.393:  "Not more than 24 hours after the conclusion of the fall state convention, the state central committee of each political party shall convene and canvass the proceedings of the convention and determine the nominee or nominees of the convention for the office or offices of justice of the supreme court. Not more than 1 business day after the conclusion of the state convention, the chairperson and secretary of the state central committee shall forward by registered or certified mail to the secretary of state a typewritten or printed list of the names and residence, including the street address if known, of the candidate or candidates nominated at the convention for the office or offices of justice of the supreme court. The secretary of state shall forward a copy of a list received under this section to the board of election commissioners of each county, in care of the county clerk at the county seat. The name of each nominee on the list shall be printed upon a nonpartisan judicial ballot containing no party designation together with the names of incumbent justices filing an affidavit under section 392a."