First, check scheds and see (1) on return, if you arrive SJU on LIAT within 2 hours of booked US departure......US has an unofficial "flat-tire policy" where they'd booked you on the next flight if you're at airport within 2 hours of flight time (2) what's the frequency of US flights out? Next flight may not be til the day after....

At worst, it's a $150 change fee to US plus more if tix prices have gone up since. From what I hear, those LIAT flights do not have the best on-time records, and you may need to give yourself more time for a stress-free connection at SJU.

Hope everything works out!