You don't mention your set sail time, but all guests I believe must be checked in 1.5 hours prior to sailing. Figuring you have a 4:30 pm set sail time, this means you must be checked in by 3:00 pm. Figure 45 minutes to Miami from FLL is definitely cutting it close. I figure by the time you get your bags, it will be close to 1:45-2:00, provided you're even on time.
I'd definitely pay the change fee and get on the earlier flight if you can.
2007 - Majesty of the Seas
2009 - Enchantment of the Seas
2010 - Freedom of the Seas