Originally Posted by
Kiwi Flyer
I don't recommend that. Not all gate agents have enough time to make the change (are you willing to hold up the flight?), or the patience to make the change for something that in their eyes is totally unimportant.
I would change at check in. You can always show your other card at the lounge to get the added benefits.
You could also check in with FTL to get the benefits of the weight limit. After then I would ask, if they can change it, because you would like to credit the miles on a different account. Lounge access with BP and FTL card would be no problem.