Follow up on what subsequently happened.
So went to check in for ET645 the next morning and I noticed that only two check in counters were open. Apparently most passengers checked in the night before and only found out when they arrived at airport that the flight would be re-timed. Airport staff (the agent that handles all Hong Kong reservations and check in operations) also surprised that "HQ did not simply notify all passengers with the press of a button". Anyway breezed through check in. Thanks to bidding for upgrade, got the comfort of lounge hopping in Hong Kong before boarding flight. The flight itself was very pleasant the the crew very professional so can't complain on the actual flight experience.
Passengers who had connections were seriously impacted and the ground staff at boarding gate were being yelled at nonstop. I feel sorry for them.