New York election law makes the term 'byzantine' seem like a good thing. The Empire State has what's called a "lock box," which is a rule that requires voters to register in a party almost a year before the next primary election -- actually the rule sets registration time by the general election, but in fact the biggest impact in on the ability of voters to vote in a primary election. If you change parties, you must wait until after the next general election to be recognized as a member of your new party.
Trump leaving the GOP?...not in a New York minute (By Kerry Picket, Washington Times) Trump could run as an independent, even if he was still a registered Republican. He would just need that party's permission and meet that state's particular requirements from whatever line he chooses to run on. For example, if he wanted to run on the Independence Party line in New York state, he would need a Wilson-Pekula (a New York legal term granting permission to run on a party's line that you are not a member of) from that party.