You must really be confused. You are not adding new lines. You are simply replacing 2 smartphones with basic phones so you can eliminate 2 data plans of $30 each. There is no cost associated with swapping phones on existing lines. Next, you want to port out 2 phone numbers to another provider. When you do this, Verizon will assign new phone numbers to your existing lines of service and there will be no charge to do that. However, make sure they don't cancel those 2 lines of service or you will incur an ETF charge of $350 per line minus $10 per month of your contract you have completed. Of course, you would not receive any discount if you purchased 2 new basic phones from Verizon and paid full retail price. If you took the discount, you would be signing a new 2 year contract.
If you call Verizon and tell them you need "two more numbers" you are going to end of with 4 numbers and 4 contracts. You are not trying to add anything. You are replacing 2 smartphones with 2 basic phones on 2 existing lines of service. Then, you are porting out 2 numbers to another provider and replacing those 2 numbers with 2 new numbers on 2 existing lines of service. If you aren't careful, you're going to screw this up and owe almost $1,400 in ETF's.