Roland,
Wow, a blast from the past
It's been a while since we heard about UShorten. I'm glad to hear it's worked well for you after all this time.
Unfortunately, as you can probably already guess, we aren't upgrading that extension to Joomla 2.5+ However, I think with a simple MySQL query, you can export your current short URLs and add them to your .htaccess file to recreate the same redirections. It will take a little work and testing, but should be possible.
So, open up a MySQL query tool, like PHPMyAdmin. Then, run the following command:
SELECT CONCAT('RewriteRule ^', short_url, '$ http://YOURDOMAIN.com/', long_url, ' [R=301,nc,L]') INTO OUTFILE redirects.txt FROM jos_ushorten_short_link
MySQL doesn't always allow the INTO OUTFILE, depending on your permissions. So, you may need to change that query, but hopefully, that gets you started.
The output will be clean statements you can just throw into your .htaccess file anywhere after the RewriteEngine On statement. They'll look like:
RewriteRule ^a8X$ YOURDOMAIN.com/long-url.html [R=301,nc,L]
Hopefully, that all makes sense and/or you have a developer you can hand these instructions off too. It shouldn't be too bad... hopefully.
Thanks, and good luck,
Alex