I am curious how it differs from Portal Fantasy as from what I can find is this: "protagonist inhabits an existing person's body after migrating across worlds. It differs from reincarnation in that the person is not literally born again." To me it seems like another form of portal fantasy much like Isekai but I could be wrong.
Topic on Talk:Alternate Realm Boon
By what I see as a common denominator in "transmigration" fantasy novels of Chinese and Korean origin, in that genre the person moves into the body and identity of another entity (usually human, but not necessarily so, as I have seen transmigration into fantasy races, into monsters, and even into plants) in the new world, leaving their former body behind; you can say that only their "soul" does the travel. You can stretch that into those "reincarnation in another world" stories where the person who "reincarnates" in a new world conserves their original grown-up memories even if they are reborn as a baby. In Portal Fantasy, the person usually travels to their new world with with their original body, and their original body receives the new boon; in transmigration/reincarnation fantasy the character receives either a new better body as a welcoming boon, or soon gets a new boon on the originally-normalish body thay they now inhabit.
It seems like another form of Portal Fantasy much like Isekai at least to me. Since it's also a trend in eastern literature. what do you What do you think?