[Samsung Galaxy Nexus] PGHammer:
Moving a GNex toro from VZW to Tracfone - all done!

Actually, you ARE wrong; I'm taking my own ex-VZW GNex to Tracfone, and it does NOT have to be stock when you do it.

Tracfone itself is a neutral (CDMA/GSM) MVNO and uses the towers of three providers (T-Mobile, Sprint, and VZW) on a regional basis; the difficulty is that it's easier to port a Tracfone phone number - which you can't do from a feature phone. This is why you need a customized SIM kit (pre-loaded with the target phone's IMEI and MEID). I'm taking a lifeline number issued by Safelink Wireless (Tracfone's lifeline brand - porting such a number is legit), and the IMEI and MEID are hardware-specific - NOT carrier-specific (the IMEI and MEID travel with the phone - and are neither carrier-specific or ROM-specific; other than the stock ROM, I have changed firmware twice, and neither IMEI or MEID changed a whit).

What you have to do is go online and explain the situation to the agent (that you are transferring your existing number to a former VZW Galaxy Nexus); the agent will ask for the IMEI and MEID of the target phone, and confirm that said phone is NOT active on VZW's network. The bigger issue with the SOURCE phone is that - more often than not, it either uses a different SIM (Tracfone sells LTE phones itself; however, they also sell feature phones up to 3G, such as various Alcatel models that don't run Android) or doesn't use a SIM at all (these non-Android Alcatel OneTouch models are in that category). As a backup, grab Tracfone's Transfer Wizard from the Play Store after the ROM changeover (it's a free app).

SIMs for phones are of three sorts - standard (the oldest Android phones, such as our GNex), micro (later, but not necessarily current, such as the Nexus 5 or 6) or nano (Nexus 5X, Nexus 6P, and Pixel, Pixel XL, and Galaxy J3) - the SIM must match the phone! I have a nano SIM for Google Fi - can't use that in a GNex; microSIMs are commonly purchasable, even for Tracfone - however, I can't use THOSE in a GNex, either. Hence I have to go custom. (Turns out I was wrong; the GNex uses a microSIM - I took a picture of it after the swap (the VZW microSIM, that is). When I have enough points, I'll attach the picture I took; it's currently backed up to cloud storage.)

When your SIM kit arrives, you WILL need the number from the source phone - with feature phones (including lifeline phones, such as the Alcatels I mentioned), this will be in the Prepaid section of the phone's menu. There will almost certainly be a "dummy" SIM in the phone you are moving; that is fine; as the Galaxy Nexus does NOT support SD cards at all, that tell you where your new SIM should go.

While you wait

Like a lot of the phones of the same age, the Galaxy Nexus can be used as a tablet while not on anybody's network; further, unlike most tablets, it supports both 2.4 GHz and 5 GHz. This lets you "explore" a bit and find the ideal ROM for you and your apps - not all ROMs , even for the GNex, are the same; between straightforward OS variety and other ROM-specific features, the differences are legion. (Not every user will be comfortable with Unlegacy Android - or even with 7.1, for example.)

My SIM Kit arrived, and it is FULLY compatible with most CDMA (and all LTE) phones from VZW. Even according to the booklets (plural - both English and Spanish), stock firmware is not a requirement. (In fact, I'm doing this with OmniROM 6.0 Security Update installed.). The surprise is that the GNex uses a microSIM (not a standard SIM) - I'll be including a picture of it in a later edit (the original one from VZW). The transfer is, in fact, nearly done; the hiccups were all on the back end - and had mostly to do with a quirk in OmniROM 6.x; the animation of the logo went a bit screwball (it kept not only repeating itself, but interfered with app recognition). It's why I put Unlegacy-Android back on the GNex in question.

If you have issues

This is actually included if the number ports, but hangs between Tracfone and the phone itself (and is referred to via the documentation) - auto-programming mode; you dial (via the Phone Dialer included with GApps) a special number (it doesn't even look like a real phone number, so you can't dial it by accident) and the phone is programmed to your new carrier (Tracfone, in this case) utterly hands-free. (It's also why even if you are running a non-stock ROM (all too likely), it's utterly irrelevant.

Lifeline phones

Yes; lifeline numbers from Safelink CAN be ported this way - in fact, Safelink is specifically covered as a number source - so you do NOT need to purchase an airtime card (either now or for at least a year - my ported number is, in fact, covered until January 2018). Be warned, though; your feature phone will basically become a paperweight.