It's a two step process. Step one change your card from visa to MC, step two reapply for the visa and get your second 50k. No need for any downgrading.
I already downgraded my SP Visa to NP (Non-Preferred) Visa. I couldn't switch to WMC because my year was almost up and I would have gotten billed the annual fee, as I assume converting from SP Visa to WMC will not restart a free first year. (Am I wrong?).
Anyway, can I re-apply for SP Visa now that I downgraded to a NP Visa?
If yes, how long after the downgrade should I wait?
Thanks!