adding phone to old account

We have an old Verizon wireless account, 3 bag phones sharing 400 minutes a month for $45. Since that is such a great deal we hate to mess with it. We now want to add another phone, a tri-mode flip phone for our daughter, but we can't add it to our plan without changing it which would then be very expensive. I was wondering if we can change one of the bag phones for a flip phone and keep the same #. Would that make us a change our plan? I'm also wondering it is possible to not deactivate that bag phone and then would we have 2 phones with the same #?

Thanks!
Lisa

First of all, you cannot have two phones programmed to the same number. You could upgrade your phones, or find some off of eBay, or another source.