Conversion from bbcode to html

Hi,

I have the default input format as BBCODE
Now that client requirement has change and it wants the full html in place of BBCODE
So i have created module for the same and have this function it which converts the BBCODE to full html
function conversion_convert_to_html()
{
$node = node_load(1057);
$rendered_node = node_build_content($node);
$node_body = $rendered_node->body;
$node->format = 2;
node_save($node);

}

the function works prefectly for the conversion but the only problem is see here is
in the description filed after conversion when node_save($node) , description field contains both BBCODE and FULL HTML as well.
I am trying to delete or replace this bbcode with the converted html but that doesnt happens.
Where am i going wrong here?
Any suggestions/ideas/help on this please?
Thanks allot!