Switching from Postpaid to Prepaid

Just want to double check the process of switching from my monthly plan to a prepaid plan is correct as the follows;

Get a prepaid SIM > activate > port old number > then the plan will be switched and I’ll be charged for the portion of the monthly plan that I’ve used (as I’m switching mid-cycle) then the prepaid plan from there?

I’m also curious as they’re having a promotion for the base plans until Feb. 16 - will my prepaid base plan stay at this price even after Feb. 16? Or is it a one-time deal and revert to their regular price after the promo expires?

any help appreciated - Thank you!