** my comment **
Agree with your thoughts. You mention that you should have a plan B in case demand is higher than you speak. I would take that one step further. You need to ensure that your front-line decision makers are armed to deal with whatever comes up. You might have, as you noted, an upside surprise in demand. You could also see demand vary in geographies differently than you had planned. There’s also the potential for supply disruptions that could impact your ability to meet the demand that the promotion generates.
The reality is that you just don’t know. Unless you can predict with absolute certainty how your promotion will impact your business, you need to be able to respond to unexpected issues. A lot of money goes into a promotion, and the last thing you want is to find that you can’t respond as needed to unexpected outcomes.
Empowering front-line decision makers with the ability to quickly sense issues, to fully analyze their impact and then to collaborate on a profitable response, you’ll go a long way toward ensuring a successful promotion.