Not sure the PNR can be split, though by the looks of it you would end up with 2 PNRs. And once you are on that separate PNR I don't think your legacy seat selection remains secure. I've had a slightly similar experience, when I moved to another flight, while a member of my family stayed on the original flight. For a period his seat selection stayed the same, but then about 8 days before travel he was allocated to another seat. At OLCI he was able to get a similar but different seat.