Who is operating your MIA-LIR flight? If it is anyone other than AA, I don't think so at all. No airline is as generous as AA when it comes to separate itineraries. Your original timings, while "legal" if they had been on a single ticket were entirely unsafe for separate itineraries in the first place and left you very exposed. I would probably try and exploit the change of times to choose a significantly earlier first flight or later second flight leaving at least 2-3 hours between the two. BA will certainly not allow you to change the routing unless there are award seat available on what you have. They may allow you to change the times on the same itinerary. If you are all on AA, AA may accommodate you if you miss your flight but I don't know how often they fly between MIA and Liberia.