It depends on who you talk to, and when the Maxx drops (as mentioned above). When I purchased my Razr, I fully intended on exchanging it for the Nexus, so I spent some time talking with the Costco (kiosk) employee about the policy. He basically explained that it was a 90 Day "Exchange for the Same Device" Policy, and not designed to allow people to swap phones (essentially a warranty). However, he also completely understood my intentions and was understanding, so he provided me with his personal number so that any exchange I made would be dealt with by him. He said that his manager would likely not understand, or approve the "exchange." Also, there should not be any fee to make the switch; it's part of the benefit of buying through Costco and not Verizon.
If you do want to exchange, and it is within 90 days, I would just talk to someone at the kiosk and explain what you want. If they say no, you can try coming back another day and talking with another employee. Worst case scenario, nobody will let you make the switch and you are "stuck" with an awesome device!