Upgrade

I am trying to upgrade my phone to the android.  I currently have the $30 plan and withthe new i need the $45 plan which is fine.  When i try to keep my number it says that i can't do it unless i purchase a $45 plan for my old phone first.  Is there any way that I can keep my same number, upgrade to the new phone and then add a $45 plan to my new phone  all with keeping my same number??  If I do have to add a $45 plan to the old phone that I will be shutting off will this plan transfer to my new upgraded phone??