The deadline for that would be the end of the promotion, so April 30. I'd wait until you're sure which one you want before calling.

Historically, it's been Plat only, but I believe there were some reports of them letting others change last year (I don't remember for sure).

If you find the MegaBonus thread here, you can cobble together a list of the various versions of the MegaBonus fairly easily. You'll probably want the one that's 25k for 15 nights and 40k for 20 nights.