There is something more to do with GSM systems and phones. If I change the SIM card the "phone" works but for data and MMS (large SMS with say a picture) we have to, most of the time, update the APN.
There's a chance in the near future that CDMA (mostly Sprint) will pass as Tmobile buys them and what happens next is in the news.
CDMA gets the most upset folk because of phones are never "unlocked." You get to deal with having it moved to the new carrier. This alone is such a pain compared to unlocked GSM phones that I learned to get away from CDMA long ago.
I am trying to better understand this process here in the US. This represents my (possibly misinformed) understanding of how things work. For purposes of this discussion assume that we are starting with a factory unlocked, GSM and CDMA compatible phone. Moto G6 or Samsung S8 if specifics beyond that matter. And further it will probably be various MVNO's that I would be dealing with here (Republic, Consumer Cellar, etc).
1) There will be no locking to start out. A GSM based plan will use a SIM card. A CDMA plan will probably also require a SIM card but it will be used only in connecting to the relevant LTE network. The CDMA provider will put me on a 'white list' that will allow me to access their network. I assume that this is a reference to my phone ID.
2) If I sign onto a GSM plan and were to simply replace the GSM card with someone else's GSM SIM card, then my phone would now work with this new provider with no other steps on my part or the provider's part. I could even move back/forth at will assuming both plans were active.
3) If I have signed onto a GSM plan, then I can also change to a different (CDMA) plan just like #2 above.
4) If I start out on CDMA, I will need my CDMA provider to take some action before I can change to a different CDMA provider and I will not be able to hop between CDMA providers by simply swapping SIM cards.
How much of this did I get wrong? Thanks.