If you cancel a Visa, it doesn't count as a close of an MC within that last 18 months. So that's good.

If you wait and let them convert the Visa to an MC, that seems to count as an open of the MC, and thus resets your 18 month clock for that type of card. So that's bad.

(All 18 month clocks at Citi are measures with respect to the last open or last close of the same type of card, and it seems to be whether you opened or closed manually or whether Citi opened or close automatically.)

So it depends how you handle the Visa->MC conversion whether it's good or bad, and the best thing is to know it's going to happen (by reading FlyerTalk threads about the cards you have) and cancel proactively before the process even starts. If you wait to learn about it until after Citi has already converted you from Visa to MC, it's too late now to worry about signup bonuses, your 18 month clock has already been reset.

And in case there are any changes to the above "crowd-sourced rules", they'll be explained on a card-by-card basis in those specific threads, not in this thread.