Is that Change just recent because Their customer services told me once i check in, Then go into the app and change from the Cathey loyalty programs to BA in order to get the avios Points ( From Cathey Customer service ) If you booking is linked to your British Airways membership number, online check-in will only be available within 24 hours before departure.

Alternatively, you may link your Cathay membership number to the booking, which will allow you to check in online up to 48 hours before departure.
After completing online check-in, you can then remove your Cathay membership number from the booking and add your British Airways membership number to ensure your points are credited correctly.

Thank you, regards returning back to the UK, is it possible once arrived in Bangkok, if we then link back to Cathy, will we then be able to check in on line for the return and then link back to BA ? If I don't link back to Cathy, then the online check in will be 24 hours.


Yes, , your understanding is absolutely correct.
If you don't link back to Cathy in Bangkok, then the online check in will be available within 24 hours only.