Help required on form personalization to modify "Order By"

Hi All,
    I added a form personalization to change default order by of one of the block.

Before personalization, the default order by clause of this block is say A

Added the following personalization.

property_name as ORDER_BY and value say B,C desc

Now after this, the order_by of this block has changed to

A,B,C desc

so, basically order_by is getting appended to original order_by instead of replacing.

How can I replace the order_by clause of the block through form personalization.

Regards,
Sirisha