I don't know what the variable name is for the userid, but I'm guessing you do since you are already displaying it so for the example I will call is $userid...
Code:
<if condition="$userid = 750">
$user_display_id = 2;
<else />
$user_display_id = $userid;
</if>
Then change wherever you display $userid to $user_display_id and it will over-ride the number for user '750' in this example with '2'