Honestly why would anyone want to complicate things?
I always put my FFP that I want my flight credited to, and then show the card with status to enter lounges etc.
Some people think that by putting their status FFP would give them the perks of that op'up. Well yes, that would help... so put in your FFP of choice and then you may try to change it ... at the gate.
Some places you can change it at the gate, some places will not allow it - like KUL / PEN for example.