Those who say Morid Hatal start saying it at musaf, as normal.
For those who don’t normally say it, the question is when to stop saying Mashiv Horuach. R Hershel Schachter has paskened that on musaf of the first day of Pesach they should say Morid Hatal, even though that is not their usual minhag. At mincha they should start their normal summer minhag of not saying anything.