index.php
, а параметры dispatch=discussion.view&thread_id=1
следует искать в %{QUERY_STRING} или %{THE_REQUEST}.RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteCond %{QUERY_STRING} (?:^|&)dispatch=discussion\.view(?:$|&)
RewriteCond %{QUERY_STRING} (?:^|&)thread_id=(\d+)(?:$|&)
RewriteRule ^index\.php$ /reviews/%1/? [R=301,L]
RewriteRule ^reviews/(\d+)/$ /index.php?dispatch=discussion.view&thread_id=$1 [L]