Today’s Question: After working on images for a while [in Lightroom] without sidecar files, when I change over to utilize the sidecar files will all of the already adjusted images have sidecar files created?
Tim’s Quick Answer: Yes, when you enable the option in Lightroom to save metadata out to the images themselves, Lightroom will immediately start updating all existing images. Thus, XMP sidecar files will be created for all RAW captures, and the metadata within the image file will be updated for other file types.
More Detail: Enabling the option to have Lightroom save metadata out to the images (instead of only within the Lightroom catalog) provides a couple of benefits. One, it enables you to see standard metadata updates (such as keywords and star ratings) with other applications used to browse outside of Lightroom. Second, it provides a backup for most of the metadata updates for your images beyond the Lightroom catalog.
When you turn on the “Automatically write changes into XMP” checkbox in the Catalog Settings dialog, Lightroom will immediately get to work creating (or updating) XMP sidecar files for RAW captures, and updating the image files for other file types. To enable this setting choose Edit > Catalog Settings from the menu on Windows or Lightroom > Catalog Settings on Macintosh. Then go to the Metadata tab and turn on the “Automatically write changes into XMP” in the Editing section at the top of the dialog.
If for any reason you want to make sure the updates were applied, you can also “manually” save the updates so you’ll see an indication of the progress on the identity plate. To do so, go to the All Photographs collection in the Catalog section of the left panel in the Library module. Choose Edit > Select All from the menu to select all photos in your catalog, and then choose Metadata > Save Metadata to Files from the menu to initiate the process of saving metadata to your files.