Migration from Drupal 7 Sites Using Relative Links

Migrated several sites to Backdrop from Drupal 7 with hardly any problems. All of the sites were "simple" in that they were only a few pages with easy to fix problems (mostly non-profit sites). I tried to migrate the first "big" site today which has almost 100 pages and nearly 1,000 links to PDF files. The Drupal site uses relative links to all the PDF files (/files/pdffiles/<file name>). Those links migrate successfully to Backdrop but the links are broken. Backdrop multi-site wants the links to be /sites/<site-name>/files/pdffiles/<file name>. The Drupal site is also a multi-site setup. I am using the instructions from the documentation site for migrating a site from Drupal to Backdrop which says to setup the folder structure, copy files from the Drupal site to the "files" directory in the Backdrop site. Modify the settings.php file to point to the database with no other changes to the settings.php file noted. Navigate to the "new" Backdrop site and run <site name>/core/update.php. Over 200 updates complete successfully but no joy on the broken links. I have tried the following modifications to the settings.php file without success:

1) Modify the trusted host pattern line to reflect the URL of the Backdrop site ($settings['trusted_host_patterns'] = array( '^<site name>$', ); . This changes nothing except to eliminate the error message after running cron the first time.

2) Uncomment the base_url line so that it points to the new Backdrop site (// $base_url = 'http://<site name>.com'; // NO trailing slash! Still, no help with the broken links.

I should note that each of these changes follows the complete removal of the failed migration instance and starts with a new, clean install. Can anyone suggest a solution? The only other thing I can think of is to use the REPLACE function in SQL (Mariadb) to modify the links.

I should also note that one of the smaller sites also uses relative links and that site migrated successfully with no broken links. I just don't know what I did differently during that migration. Any help is appreciated!