It will be more expensive post booking. You will not be able to change from value to regular fare, but you will be able to add each of the services, but overall it will be more expensive. Anyway, I follow the same strategy as you suggest. Pay the absolute minimum when the booking is made and then add services once it's clear that I will use the ticket and what I need (I often travel with small bag only and I never pay to select seat or for priority boarding; I simply don't care about this).