I DID read the entire thread son. Only thing I could ascertain is that you haven't used this trick since January because you have the mastercard variant as well. You asked someone more knowledgeable than you if he was able to change his DC variant, to which he replied in the negative. STILL NOT YOUR EXPERIENCE. IN FACT you asked the SAME question:
You DID issue a blanket statement though:
TWICE:
which cannot be corroborated with any other source as the PDF of T&C doesn't mention the card variant anywhere and nobody else has confirmed OR denied the MC variant cashback eligibility.