Want a iPhone 5s but not sure if I can get it

In November I get a upgrade from my phone to a new one and I'm switching from AT&T to a line on my moms plan on Sprint.
So I was wandering since I'm switching to sprint (but already a person with a plan) am I able to get the iPhone 5s 16GB for $99.99 or am I going to have to pay full price?

The best logical way is to contact the both carriers for their regulations, policy, and contexts of their contracts?!