I've found that sometimes there are long lines for the ATMs...but rarely for the forex counters...which are open 24hrs a day. Rates are worse than the banks, but for a short trip, probably makes little difference. If you have time in your transfer at PEK, go ahead and change there, but I think 2hrs is a fairly tight transfer window: I would prioritise getting to my flight on time! I can't recall if there is a forex window on the domestic departures airside at PEK: I don't think there is at T3, can't recall from T2.

tb