+ AA6YQ comments below

OK, I figured out how to do this with some fancy footwork using the log's backup. But it sill would be handy to know how to undo that operation.

+ The "Modify QSOs" panel's documentation clearly indicates how to be able to undo that operation:

https://www.dxlabsuite.com/dxkeeper/Help/ModifyLog.htm

https://www.dxlabsuite.com/dxlabwiki/QSOModifying

73,

Dave, AA6YQ