I was in a similar situation when I was not in HK during the call up phase in 2003. I eventually renewed my ID card this year when we were relocated back to HK. You have within 30 days from the day of arrival in HK to renew (although if you're on a short visit to HK then you can apply later which I did). The renewal process was quick and no appointment was required. Collection of the new ID card takes 2 weeks.