You can't use those phones until they are out of contract. You would finish the contracts then ask to move from month to month to prepaid and take your numbers with you. There is no downside for me...coverage is only 10% less countrywide, and the data is enough for me. I am on the $45 2GB (autopay) plan. I pay $50.80 with tax a month.
In multi line situation, post paid can be cheaper sometimes, so make sure you are making the best move.
now that you will have no contract 3 lines 6GB data is $120 on My Verizon plan, or $40 per line.