Well, that is very unfortunate. The behavior to remove tags however is not limited to this extension, as it turns out it's a Firefox bug that whenever any extension modifies a bookmark URL then all its tags are removed. Here is the relevant bugzilla entry for that: https://bugzilla.mozilla.org/show_bug.cgi?id=1440988

You can test the regexp mode to change just the bookmark title but leave the url fields empty - in that case the urls won't be changed and thus the tags should be preserved. Unfortunately this won't help you update your http bookmarks then, sadly it looks like no extension can do that while preserving tags.

But I need to add this information into the extension UI - so thanks for the heads-up.