The "special" B fare for non-PremPlus flights on a PremPlus fare is only for UA flights. Other carriers book into different fares -- have seen as high as Y and as low as Q. And varies depending on the PremPlus fare class).
Best to mention if you are included partner flights because that can change the situation.
Addition TA issued tickets have different rebooking rules than UA issued ticket, so also worth mentioning.