RewriteMap subtract prg:/path/to/script/that/subtracts-by-one.sh

# to make sure we aren't clobbering legit requests
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{QUERY_STRING} aid=([0-9]+)
RewriteRule ^/?index.php$ /articles/${subtract:%1}/? [L]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{QUERY_STRING} aid=12345
RewriteRule ^/?index.php$ /articles/12344/? [L]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{QUERY_STRING} aid=13337
RewriteRule ^/?index.php$ /articles/13336/? [L]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{QUERY_STRING} aid=19911
RewriteRule ^/?index.php$ /articles/19910/? [L]