Looking for some advice on the best way to upgrade my phone and Broadband PC card on Verizon from 3G to 4G/LTE.

I have unlimited data plans (grandfathered) on both a 3G PC card as well as unlimimited data plan on my 3G Droid3, I have been told that I can carry over my phone unlimited plan to a 4G device and am tentatively going to do this when the Droid 4 is released (I am partial to a physical keyboard). I tried to do the same with my PC card for a USB or MiFi device but despite the fact that the Verizon rep said the billing codes are the same, she had a screen pop up that prevented her from keeping my plan and shipping me the 4G device. I have heard about moving SIM cards to MiFi devices, and am open to any other options. Thoughts on how to migrate and stay unlimited on both devices would be welcome (tethering, SIM card migration, other?).