Since that post quoted my original post on the BAEC forum I have to add that my understanding have since changed a little since that post (as reflected in my above post in this thread) -

Primarily that 1) IFM and CCs can update their iPads whilst on ground (I got a really odd stare from the IFM on the above-mentioned BKK flight when it went from having the EM flag on my profile to nothing from across the cabin) and 2) I have had the "flight departed" message a bit later at HKG and various outstations (especially when there are ATC delays even after doors have closed), so technically you are able to change the FFP till that point.

As it stands though, I tend to find changing onboard the best practical advice if one wants to maximize their OWE/DM benefits whilst crediting to another FFP on CX. But as I have had the flight departed message appear almost immediately when they announced "boarding completed" - with boarding completed happening really shortly after it started before I do think it's very much getting the timing right, but I'll generally err on the side of caution.