On the off chance that you're using Apache and can edit /etc/httpd/conf/httd.conf (or the equivalent), something like the following might work:

  RewriteEngine on
  RewriteRule (.*)     https://%{HTTP_HOST}$1      [R]
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You might get by with putting this in a top-level .htaccess, though this might also require adding

  Options FollowSymLinks
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(At least that worked once for me, after wresting with a cryptic error message.)