Being based in Asia/HK now I'd suggest that it is unlikely you'd be able to change the flight. My logic is that even on revenue tickets the general practice from CX agents would be to only move you to an earlier flight if you pay a change fee or per the ticket conditions (e.g. I've had them decline paid upgrade requests as my ticket was supposedly noted as not being upgradeable and ask for a $100 change fee for a cheap economy ticket even if the alternative was me hanging around Singapore for an extra two hours as a oneworld Emerald, which I ended up doing)

HK airport is nice enough so perhaps not the worst place to hang out if they decline your request, but the city itself is probably a more fun place to spend time than PEK . You may also get 'lucky' and have your flight to PEK delayed or cancelled (due to smog) so you'd have more flexibility on flights then