I disagree with the idea that the Norhternmost part of New England has trended Democrat only because of migration, since other Northeastern states are just as liberal as before (MA, RI, NY). At the same time NJ and CT have also trended to the Democrats.
NY has become even more Dem & liberal than it use to be. The NYC suburbs usually kept the state at least within reach for the GOP, but has trended to the left starting in the early 90
s. Casing point Nassau Co (where I live) went Republican in every Pres election from 1916-1988 except for 64 when LBJ won every NY Conty, it has gone Dem each of the last 4 Pres elections. Westchester has gone through a very similar shift, as has Suffolk County. It may not have that much to do with migration, but just the overall idealofgical shift of the region as well as a growing shift between the two parties on social issues
