RewriteCond %{THE_REQUEST} ^GET\s /\s?(([^&\ss]*&)*)tag=([^&\ss]+)&?([^\ss]*)
RewriteRule ^(.*)$ /tag/%3?%1%4 [L,R=301]
Then you can rewrite that requests back internally without conflicts:
RewriteRule ^tag/(.*) index.php?tag=$1 [L]