I've hung out with a bunch of Mormons and while they personally felt it was weird now, because they have grown up in a monogamous nation, the church itself would definitely gear up to switch back, if it was legal.
That would be a process, though. They are currently taught that it is morally fine, but following the laws of the land is important, and basically treat it like part of their history. On an individual level, the ones I know seemed fine with that, even those that had an active role in the church.