Sorry about that.
What we're doing to fix this:
I'll be creating a new page that will allow you to do this and bypass the refund/rebuy manual route CSRs had been providing.
This way, you'll just pay the difference between the Masakari and the A La Carte collection you have, if you want to upgrade.
This is primarily of interest to people who bought a large number of A La Carte Mechs and want to qualify for the Mad Dog in addition to all of the other free mechs we're giving out.
Again, apologies for the inconvenience to any A La Carte owners who want to upgrade.