The easy way to do this is by creating a replacement variable (under Styles & Templates).
For example, with SEO turned off, you can make it rewrite
https://yoursite.com/forum/member.php?u=1
to
https://yoursite.com/member/1
But it depends on the structure of the URL. What does it look like?
It might be necessary to make modifications to the function that creates the SEO friendly URL.