you say 而家有公屋等於無公屋住=> As the 公屋 is yours (in your name), then why you move out ....... it should be the 男家 move out.
Also, if you buy the 私人樓, you should give the 公屋 back to the gov., right?
Then, you should use your 公屋 to buy (change) for the 居屋. It is 30-40% lower than 私人樓.