We don't know who your resources are.  You might not be asking how to change a SIM, but your question involves the use of SIM cards and alternate devices as they pertain to plans, which you base your info on in regards to FCC information.  You asked is the info correct, I just basically told you, YES...a customer can keep their plan so long as they do not sign another contract which offers a subsidized phone.  Changing plans changes plans.  Otherwise, with the BYOD (compatible) idea, or the buy-at-full-price or buy elsewhere ideas, one does not have to change plans.  You put your SIM or one that fits in the phone and you keep your plan. 

.....The SIM holds the detail of your data plan  No, it doesn't.  It holds your phone number and access to the network.

.....how to help someone keep unlimited data with a renewed contract.   You can't keep unlimited if you change plans.  If you change plans you cannot go back to a plan that doesn't exist. 

.....independent of hardware or software questions.  A SIM is part of the hardware and your initial question involved user data on that SIM.

You want validation of already widely-known information.  Without quoting and responding and pointing out the obvious, YES.  IDK where this is coming from, but suffice to say, I speak the truth.  Any other responses should be aimed at the link above that I posted to leadership.