To add a data point, my mother just had her 30A seat swapped just before she checked in (she was forced to check in at airport as there was no other way of getting her IB Club number entered into the booking) to 30H on today’s CX372 MAD-HKG

Perhaps best to OLCI (without adding/changing the FFP) to 'secure' the seat and later add the FFP in person at the airport counter

For you and me, perhaps, but not for my technophobic 77 year old mother. She was at check in way before it opened and the switch to 30H had already happened. The entire PEY cabin was fully booked, as was J. GIven what is happening in the Middle East and how demand for CX seats from Europe has rocketed, the “Priority Seats” at the bulkhead are the only wiggle room for a CS Rep needing to force a switcharoo to accommodate someone more important to them. She still got a bulkhead seat and it wasn’t in the middle, but it’s worth knowing that this can happen and maybe someone else can avoid it by checking in earlier.

(As an aside, the lovely - but apologetic - lady at check in couldn’t add her IB FFP either, so my mother’s focus and persistence went on resolving this issue, which apparently happened at the boarding gate)

Her IB number has 9 digits?

Yes.

Sadly, this was just the start. Her flight was on time but she was offloaded her connecting flight before she arrived and her fare code switched from E to L, she was prevented from following the CX rep at the plane door with her departing cx488 flight number on a board, not given information about the flight leaving 30 minutes after that and then put onto a much later flight, right at the back and middle of the plane. A 77 year old traveller on her own. The only concession were a couple of meal vouchers she struggled to use (one she gave away to an airport worker). A formal complaint has been made, I am fuming!