First off, take a copy of all of your templates. All too frequently do placeholders and templatecells get lost or corrupted, which can cause programs to not function properly or sections of published pages to be missing entirely. Your backups can also be referred to for reference.

After backing up the templates, feel free to make a few small changes. Try to test your templates often: if your placeholders have been corrupted, you might have to go back to your latest backup. Copy your templates frequently while you are working on them so you don't lose too much work if things go wrong.

The best thing to edit any template files would be a text editor, but any other HTML editor should work, presuming that it preserves comment entities. If your HTML editor looks at

<!-- template insert : $list$ -->

and decides that it's irrelevant, you may have some problems. Most good text-based editors will show you where unknown tags are and preserve their position.

If you do decide to use a special HTML editor, try to open a template file, make a really quick change, save it, and see if the program still works correctly. If this test works, you should be fine.