Can I decide what frequent flyer number I want to earn Miles on after I have checked in or travelled?

No. You must decide which programme you wish to earn Miles on at the time of reservation or check-in for Qualifying Flights and at the time of paying for other qualifying goods or services. No changes can be made to the membership number once a member has checked-in for the first flight of an itinerary.

Related questions

Can I make a booking online with my credit card if I am not travelling myself?
How do I move up to a higher tier?
When I move up to a higher tier, how long do I keep the benefits for?