The disposed property was purchased many years ago. The value has appreciated a lot. Should I be worried the IRD may impose tax on it?

While it is not clearly stated by the IRD, holding a property for a very long time is generally seen as strong support of not a speculative transaction.  However, the length of holding period is one of the facts for IRD to determine if a business has been carried on. You should be prepared in case a questionnaire is issued by the IRD.

Reference: https://www.ird.gov.hk/eng/faq/pty_tran.htm