Not really a good way. The phone will keep unlimited without issue.

The old unlimited plans on mobile broadband devices will be an issue. She actually was mistaken in that 3G and 4G plans are different for mobile broadband devices. The unlimited will only work for 3G devices and even then I'm not sure it will work on all devices. Coding is even different for USB devices and MiFi's. So while unlimited might work on a 3G USB device it may not on a 3G Mifi. I really can't think of any way to keep unlimited on that line. It's true that if it were a regular 4G phone line with unlimited data you could take out the SIM card and put it in a 4G mifi and use the unlimited data. The issue is, there is no way to turn it to a phone line and get unlimited data on the line either because that feature isn't available anymore if the line wasn't already an unlimited phone line. You could just get rid of the mifi line and move your phone sim card to a mifi and use the unlimited that way, but you'll be stuck only having one device at a time that way. You could also get a 4G device and then add the $30 unlimited hotspot to your phone since you already have unlimited data. The only issue then is turning the hotspot on and off on your phone when you want to use it.