When you get a new phone, you don't need to "port" your number if you are staying with a Verizon phone - just activate the new phone with your current number. Call customer service (prepay - (888) 294-6804) and have them activate the new phone.
The activation fee depends if you are on a Daily or Monthly Prepaid Plan. The activation fee for Prepaid Daily Plan customers is $25. The activation fee for Prepaid Monthly Plan is $35.