Right now, with the sole exception of Metro PCS, which has limited nationwide coverage, there is no major carrier besides Verizon that offers LTE in prepaid. All other forms of 4g are basically hspa plus (Tmobile and Att) plus Wimax (Clear and Sprint), both older forms which are probably going to be supplanted and replaced by LTE on those respective networks eventually.
You can't unlock the Novatel device as it is locked to Verizon cdma/LTE network. It can be used on Verizon postpaid and prepaid and that's about it. If you are looking for more flexibility in prepaid broadband, gsm prepaid modems with Tmobile and Att bandwidths might be more up your alley, though Idk if mifis have the ability to accomodate multiple networks even if they run off of gsm networks (sim card networks). Plus most unlocked broadband sim card/gsm devices are limited to 3g and not 4g. Often you get limited network choice and 4g or more choices and 3g.
It should be possible to switch between Verizon prepaid and monthly data plans but if the Hotspot device was a prepaid device to begin with , there may be a waiting period before you can put it on a monthly plan. I recommend you check with Verizon prepaid customer care to clarify that one.