I have the iPhone 6 and want the 6 plus?

So I got the iPhone 6 last October 2014 and now I want the iPhone 6 Plus. Could I switch even though i haven't had it for a year. It's the same price as the one I have now. I want the iPhone 6 Plus 64. They are both 25.00 a month. And if I can switch could I give it to my sister, right now she has the iPhone 5s 16 gb

Yes you can always pay for whatever the cost is for a 6+.

No, you have to pay for the whole plan you have left, before you can switch