I'm currently transitioning from British Airways to Cathay Pacific for FF miles/status.
I should make CX Silver (lounge + seat selection when flying CX) with three bookings, which is all I need, and as luck would have it, I have occasion for three such CX bookings in the next few months and going forward, I imagine the bulk of my flights to be CX.

I'm currently Oneworld Sapphire via BA and, as such, would like to choose seats in advance and use the lounge, etc, while on these three bookings.
When I made the first booking, I was told by a CX agent this would be no problem - I could accumulate on CX while taking advantage of BA's Sapphire status for lounge and seat selection.
Now I'm being told differently as I try to apply my CX number after booking.

The agent on the phone explained I could update the number online or at the check-in counter on departure, but I would lose the benefits for the return flight.
It got me thinking: what if I updated the number at the check-in counter on the return flight instead?

The agent confirmed this would work, but I don't have a lot of confidence in CX agents at this point.

Has anyone done this, and did it work? Are there any issues I need to look out for?

BA Gold here, now crediting to QF with CX with the recent changes, and have done it 15 times with another 9 more to come in the coming days.

CX is easy for doing this as you can indicate you want to credit to different FFPs for different flights in your PNR through their website and their app. This comes in handy for our cases since you can always switch the flight you're going on to each time, and won't lose your benefits.

The easiest way is to use the CX app - go into My Trips, click on Frequent Flyer Programme for the upcoming flight. Put in the new FFP number and make sure you uncheck the "I want to use the same frequent flyer programme for all flights in this booking", so it'll only stick for the upcoming flight. Rinse and repeat each time you need to do it.

As a rule of thumb you can change your FFP until boarding is complete. So, I tend to do the switcharoo when in my seat after using the lounge and using F boarding - this also mostly ensures that my BA OWE status will be reflected on the cabin crew's iPads which gets you first dibs on food choices and ostensibly better service (unless they refresh it again before departure) while I ensure that it gets correctly credited to QF.

Fantastic...just the sort of anecdotal experience I was looking for.

Thanks MeltingAlf

Just to add: Make sure you set a reminder to change when you�re on board! I forgot once and lost a bit of status points to BA! Not a lot but a thing to remember!

That's definitely true! I haven't forgot (yet, and I'm pretty much almost there to QF Plat so i'll stick to that once I get it) - honestly, if you're BA Silver I'll just set it to the intended FFP in the lounge and leave it at it, and board with a paper BP with the BA status on it. The onboard status benefits on CX are ancillary at best.

Thanks. This is definitely a concern; I am looking for a good way to set an alert on my iPhone.