Did you enjoy the CPF housing grant when you bought this resale HDB flat in April 2009? If you never, you might be able to buy private property and rent out your HDB flat earlier.
You will be able to buy a private property and rent out your HDB flat once your resale HDB flat fulfill its respective MOP which might be in case of 5 years.
If you have outstanding HDB loan by then, you are only eligible to loan up to 60% of valuation/ purchase price (based on current MAS guidelines)
Feel free to contact me if you have any doubts.
(M): 9111 5171