I could be wrong here - please correct me if I am - but I think there is only one case in the history of the Eurovision Song Contest where the performer of a song had to be replaced at such short notice that there are different singers in the preview clip than live on stage.
Nicole and Hugo had won the 1971 Belgian national final and made a breathtakingly charming video clip, when Nicole fell ill and found herself unable to go to Dublin.
Nicole & Hugo - Goeiemorgen morgen (Belgium 1971 preview)
Instead old-timers Lily Castel and Jacques Raymond stepped in at short notice, famously rehearsing their choreography on the plane for Ireland. When arriving at the Gaiety Theatre, they realised the stage was too small for their act anyway, resulting in a somewhat drier, stiffer version of the song. Despite having many supporters beforehand, the Belgian song found itself in a lucklustre fourteenth place.
Jacques Raymond & Lily Castel - Goeiemorgen morgen (Belgium 1971)
In all fairness, the stage would have been too small also for the flamboyant steps of the original couple but maybe Nicole and Hugo had been able to add that final punch and extra bite that the performance would have needed to convince the juries. It is a very fine song, in a brilliant arrangement, and a shame that it is somewhat forgotten in the eurovision archives.
Nicole & Hugo would get a second chance at Eurovision - two years later they represented their country with the true evergreen "Baby Baby", landing in a highly undeserved last place. I think these two were pioneers within Eurovision showbiz - talented, funny, good-looking and daring - and they would deserve so much more respect for their craft.
Hats off for both of them!
Nicole & Hugo - Baby Baby (Belgium 1973)