take it for a ride and see. some would look at me and say i should ride 60 or 61, but i ride a 58 with a longer stem and love it. bad positioning can mean pain, though, so think about the long run when you try it out. going 58 to 54 is a pretty big jump. i'd say if you're not 150% satisfied with the position of the 54cm bike, wait the extra two months to make sure you can ride the bike comfortably for years.