@akung yes, just be aware that a new tone was set for promo plans with the recent Freedom Mobile-targetted promotion, which was only being offered to people porting in a phone number from Freedom Mobile at the time of activation. This recent promo plan bucked the previous norm of always being availble to anyone, including existing customers or people coming in from any other carrier.
From this recent history, we definitely cannot say that you will be able to do this type of scheduling a plan change to a promo plan, as you are talking about, and as was always an option in the past. For at least some of any upcoming promotional plans, I think we can expect that they will have limited availability. Some could still be open to existing customers, but maybe not all. We don't know, and weren't really told either way when the community oracles asked PM about this recently. We got a fairly "political" answer, which basically boiled down to they will do what they think is required to remain competitive, and that it could be a mixture of different promotions at different times, depending on market conditions and offerings from competitors.