A Verizon lady in-store told me I could get a new line for my son, get a new (cheap) Rezound with that new line, then when I activate it go into the store with the Rezound and a cheapo phone- (She said it doesn't even have to be a data-phone on the new line, and the example we talked about was getting an LG 8300 flip-phone off eBay or from a friend) and activate the new Rezound on my wife's unlimited data line, and the LG 8300 on the new line (for my son.) If I do that, I'm getting a $250 phone for free, and spending $10 per month on the new line: That's a no-brainer.
Now, what I'm suggesting seems a little "marginal" to me, in that they *could* say, based on the contract language, that getting the new phone requires getting a new data package, and I would no be going that when the phones are actually activated. But Amazon wouldn't know that when the phone is ordered, and it sounds like VZW doesn't really care to tell them.
Now, what you are proposing is similar to what I want to do, in a way, except that you want to just cancel the old lines, rather than activate a phone on them. It seems like if your old line is two years old, you *should* be able to do what you suggest without penalty. Note you will lose unlimited data, because the new lines won't have it grandfathered.
Where I think you might run into trouble is: 1) Changing the phone number over. The phone number is a property of the new line when it's created. I don't think you can change the number to an existing line's number, and you can't request a number that is already in use by another line. 2) There may be some difficulty in cancelling your old "primary" line, leaving yourself with just secondary lines... I would *hope* that the system is capable of promoting one of your new "secondaries" to be a new primary line, but... You never know. If the primary line holder was a hubby, for example, who passed away, would VZW be able to cancel his line while leaving the wife & kids in place? Probably. So I *think* you probably could too, which means you could save some $$ doing what you are suggesting, but in exchange you loose the phone numbers and unlimited data.
Of course, I'm just some random dude online applying logic to a situation in which logic may be trumped by the Process or the System. I have learned in life that the Process and the System are often far more powerful... ;-) You nay be best off actually going into a store and asking a rep about what you want to do.