Originally Posted by orbitmic

Absolutely, splitting an itinerary to do that is very straightforward and usual (be it to upgrade one, to change the itinerary of one, etc). The reason KARFA and cws say that this won't work here is only because the op precisely does not want to split the booking as they want the other passengers to benefit from the extra luggage allowance related to their own status. If the booking is split, that will be lost. It's a bit of a "eat your cake and have it" situation.