There is no early termination fee if you switch devices, only if you cancel your contract. Activating a different phone on your line doesn't cancel your contract. It doesn't cost you a thing. And with LTE SIM cards, all you have to do is put your SIM card in your new phone, and you're good to go. Quick and easy.
And, on a bit of a side note, while you might be able to get around $450 for a MAXX right now (probably a bit less actually), that won't last long. The prices on Android phones drop quickly. In six months, you'll be lucky to get $250 for the MAXX. If you want to get a decent return on your phones, you have to sell pretty quickly while they're still relatively new and in demand. Selling quickly also keeps you on the latest tech, which is fun.