Confused second-time CX flyer here.

I'm on CX872 tonight (technically tomorrow morning), HKG-SFO in premium econ. CX app let me check in but refused to issue me a boarding pass. That seemed strange so I checked flightaware and it was telling me that the plane was a 773 (it was scheduled on a 77W). I was wondering if this was an equipment swap / involuntary downgrade issue (no way I'm sitting in a regular econ seat for 12-15 hr -- I simply don't fit). I tried to call them but the call kept dropping while I was on hold. I checked EF and it's still saying that it's a 77W.

On a whim I tried the website and it happily emailed me a boarding pass with my originally selected seat in premium econ. So now I'm just confused.

Does anyone know which plane this actually is? Am I probably good to go since I have a boarding pass with a seat in the correct cabin? Is there a more reliable way than phone to contact CX? Would a 77W to 773 change cause downgrades? (I'm not sure what the cabin sizes are)

Thanks in advance for any info...