I have a year left on my AT&T contract for my Note 2 but I want to switch to the Nexus 5. The Note 2 has been slowing down a lot lately. Crashes a lot, screen rotation almost never works, and is just sluggish. If I buy an unlocked Nexus can I just take the sim card from the Note 2 and put it in the Nexus? What does this do to my contract?