When I changed my main FFP from CX to JL, I had to change FFP no in the existing reservation.

I called CX, and asked change of FFP no. An agent said 'done'.
However, still I saw my reservation under my MPC account.
In this situation, checkmytrip.com showed both of CX and JL FFP numbers.
Yes, as an agent said, JL FFP no was added to my resrevation.

I called CX again, and explained the situation.
She 'PROBABLY' removed all FFP no, and add JL FFP no.
Good. Then, I didn't see my reservation in my MPC account.

So, you have to make sure, old FFP no. has been completely detached from the reservation.