Will my parents cell phone bill go up if i only upgrade my phone?

We were all going to upgrade our phones since 2 of the 3 were broken but my dad got a warranty so they're just getting replaced but i still want the Xperia Z. We have tmobile and it says pay now like 160-170 im fine with that but it said new monthly bill like 102$. our monthly bill is like 250-262ish for all 3 phones. We have a smartphone plan which consists of unlimited call,text, and 2 or 4gb of internet usage without wifi (not a big problem since none of us really use that much) I want to know if i get the xperia will the bill jump up to say 300$ because i want a new phone and im fine with paying for the upgrade i just dont want to be in trouble if the bill skyrockets