In newer versions of CMOD, you can add a field to an existing Application Group, but that field will not become populated for previously loaded data, leading to strange situations when it comes to searching...
If you do a search that includes the new field, it will only find new documents, as the old documents have no value to search against in the new field. Some of the old documents might match the search criteria, but that index data isn't available in the database, so it will be excluded from the results -- which your users probably aren't expecting.
Do a lot of testing in your Development regions first!
-JD.