It isn't a given that the reefs will move north. Migratory fish, certainly. And in some regions some coral species are redistributing poleward, like tropical Po

It isn't a given that the reefs will move north. Migratory fish, certainly. And in some regions some coral species are redistributing poleward, like tropical Pocilopora alicae into Sydney Harbour. The pelagic larvae of some species will probably find hard substrate to settle on further north and colonies will probably grow, in some number at least. Whether there is sufficient hard benthos that isn't exposed to too much eutrophication, wave action, hurricanes, turbidity from coastal development, disease outbreaks et cetera on a regular basis for reefs to grow is another question. Keep in mind that biogenic reefs have formed over thousands and thousands of years.