I'm looking to upgrade my headjoint sometime in the future. Does anyone know whay I could do if the trial headjoint that comes in the mail doesn't fit my flue?

If it's too small, use some teflon tape (availabl at any hardware store) to fit it. If it's too big, you'd have to have it fitted by a tech, but this is not something you should do until AFTER you have purchased a head.