[telecom] What is the actual technical reason that would explain why T-Mobile is trying to get legacy Sprint customers to swap SIMs?

I've had a family plan account for several years from Sprint, which was acquired by T-Mobile last year. They have been gradually combining operations, and now I am being urged (though not required) to "Upgrade to our bigger + better network at no extra cost" by swapping the SIM cards in my account's phones for new T-Mo SIMs. The online chat agent was predictably useless in answering my questions about why such a switch would be necessary (such "why can't the T-Mo towers simply recognize legacy-Sprint SIMs in addition to recognizing T-Mo SIMs?"). Given cautionary tales such as

... I am reluctant to try to fix something that isn't broken. The fact that T-Mo historically has offered inferior coverage in my local market (though I recognize that this may have changed) is also giving me pause, as I cannot determine whether a new SIM would give me access to *more* towers (all the legacy Sprint ones, plus all the T-Mobile ones), or instead merely give me access to a different set of towers (i.e., excluding the Sprint towers I am using now). Can any Digest readers who are knowledgeable about this issue share what they know?

Thanks,

Bob Goudreau

Cary, NC