Thanks for the replies. I did try ctrl-z, but it didn't work. Upon reflection, I think this is because the original operation took some time to execute, and I should have waited for it to complete before trying ctrl-z. Could that be a possibility as to why it didn't work?
In the end, I did manage to reimport everything eventually. Thanks to the sidecar files retaining the old filenames, everything was as it should be. I think a dialog/confirmation box may have helped during my transition period getting used to the new system. The ironic thing is that I've been meaning to suggest that copy/rename/move work exactly as it does now (remembering your last setting) for a couple of years now!
Anyway, lesson learned. No real harm done. I think the confirmation of action might not be a bad idea (especially if we can turn it off), but in this case I may just have dismissed it anyway.
On a side note, I do often use the update only feature, without getting MC to move files from one "directory to another". This is because my unraid server has 20 hard drives and multiple user shares that span many drives and vice versa. So moving a file from the I:\ (Import) directory to the M:\ (Movies), for example, would require a lengthy process in MC/Windows, where the file is copied from one to the other. However, I can just go into Midnight Commander in a telnet session to my server and instantly move the same file(s) within the Import and Movie shares of the same drive. So I often shuttle between the different modes of rename/copy/move.