Hey, this is bigapplegreg. I'm posting this with my other legacy account for this support community. I'm doing so because I wanted to log into it to see if it still worked — it's this account that was duplicated when I created the T-Mobile ID I wrote about in my post above. What happened was, when I created the T-Mobile ID, I submitted the email address I have on this legacy account, without realizing it.
Anyway, I contacted T-Mobile help on Twitter to ask about the anomalies I experienced on my.t-mobile.com. I added that I thought that if T-Mobile help deleted my my.t-mobile.com account and then I set up a new one for the same phone number, I would get an "up-to-date" my.t-mobile.com account. But they looked into it and found that the my.t-mobile.com account that I have is the one that customers on the Pay As You Go - Legacy plan get, and the only way to get a different my.t-mobile.com experience is to get a different plan. (I didn't change my plan.)