Many thanks Charles - I'd missed that copletely. When I reset that image to Camera Raw Defaults, I was then able to assign a keyword without problems.
Martin Evening's book on way from Amazon
There are several ways to go back/edit history.
In Bridge, select the photo you edited in ACR. You can see the Edited symbol above the photo upper right.
Go to Edit menu, Develop Settings, Clear Settings.
In ACR you can also make Snapshots of different times in your editing session.
Go the Snapshot tab (the rightmost tab) in ACR. Click the New Snapshot icon in the bottom right and name the snapshop. You then can go back to a previous edit by simply clicking its tab. The caveat is that you need to manually add these Snapshots as you go along.
You can also go back to an earlier edit and Open the Image in Photoshop while still retaining the later edit shapshots in ACR
Lighroom has a real time history that you can go back to a previous edit. This history is automatic. I'm not sure if the history is retained after the file is closed and then reopened. I tend to think not, but you can also create Snapshots in Lightroom.