11 Jan Nerd Alert! Cannot Be Undone Warning
In Lightroom or Photoshop, any time you get a message telling you that the ensuing operation “cannot be undone”, it simply means that you can’t go back using the File>Undo menu command or the undo keyboard shortcut (Command-Z on a Mac or Control-Z on a PC). You can always revert, reinstall, or redo using other means.
For instance, when updating a Lightroom catalog, Lightroom makes a copy of the current .lrcat file, updates it, and names it with the current catalog name followed by a number. The new, updated catalog steals the preview files from the old catalog and converts them for use with the new format. Once finished, you can’t “Undo” the process using File>Undo or CMD/CTRL-Z, but you can still open the old catalog using an older version of Lightroom. You’ll have to generate all new previews if you didn’t make a copy of the original .lrdata (Previews) files beforehand, but it can be done.