It seems that your intention is capital in nature at the time of the sale of the property as you will buy another property for leasing (I assume your new property will be let out not less than 1 year). However the rental income from the old property should be subject to the Property tax.
Pls. refer to this case from the BOR
www.info.gov.hk/bor/eng/pdf/dv22_first/d1707.doc