On phones, if you pay the full retail price instead of the subsidized price from Verizon, you can keep your old plan and just upgrade your phone. I'm not 100% positive, but I believe I've heard that you can replace an old phone with a different one purchased elsewhere(as long as it's a Verizon supported phone) and still keep your grandfathered plan. Actually, you can go online and do an MEID transfer yourself which transfers service from your old phone to your new one.
It would seem logical that you could do the same with a MiFi/Jetpack.
It's certainly worth a try. BTW, if you call and don't like the answer you get from Agent A, try calling again and talk to Agent B . . . you'll probably get a different answer!
The "catcher" might be upgrading from 3G to 4G.