If you have an old BNO passport, just renew it at any time at any British embassy. You can also do this in HK, at the British Consulate-General. Check their site: www.britishconsulate.org.hk
BNO nationality can be retained for life, but you have Canadian nationality...why bother about BNO.
If you were already HK resident of chinese decent before the handover in juni 1997(because you hold BNO status)...it could be that you have aquired Chinese nationality from the moment of handover. Check the HK immigration site about this subject: www.immd.gov.hk
A HKSAR passport(Chinese nationality) is nowadays prerequiste for a 'Home Visit Permit' application at the CTS. With that permit, you can enter Mainland China visa free(saves same hassle),thats all.
@ontheroad: Just holding a HKSAR passport alone, you cannot enter mainland China, but it makes you Eligible for a Home Visit Permit, with that you can enter China visa free for its validity of 10-years.