Am travelling pretty soon SNN-BOS-LAX, booked through UA with the SNN-BOS leg operated by Aer Lingus. My question is, will I be able to check in on US.com at t-24 (24 hrs before the EI flight) for at least the BOS-LAX leg in order to secure the coveted upgrade to (very limited) availability E+ as Premier Silver? Thanks in advance for any advice or guidance.