The difficult part here is to know what changes have actually been done to the schema so you can undo them. There is no magic undo button, but here’s a working procedure to remove any unwanted changes.
- Go to site settings for your site collection
- Click Search Configuration Export below Site Collection Administration
- Open the exported xml file in your favorite text editor, as this file will contain only the changes made at the site collection level
- Look for the SearchSchemaConfigurationSettings node
- In the ManagedProperties nodes you will find new managed properties which have been created, in the image below a new one named MikaelKnowsHisStuff
- Re-crawl content to weed out values from the index
Unfortunately it’s not a simple undo operation, but at least possible. This is also why I strongly advocate to re-use the reusable managed properties for your custom use, as it’s quite easy to see any mappings done in the UI, as long as you know which ones might be globally mapped.