You can always make a booking and then ticket it later. How long that period is depends on the ticket type: Sometimes you need to ticket it by the end of the day, other times you don't need to ticket it until 24 hours before travel.

Unfortunately, the convoluted process the previous posters are referring to is precisely what logical customers have to do. Because, if you look at Cameron38's question, the reason you aren't advised to do the abbreviated 3-step process he asks about is this: the availability of the upgrade you call about in Step 1 may no longer be there in the 15 minutes or so that will elapse between Steps 1 and 3.