As an aside my train from Farringdon to Paddington was an old fashioned circle service, heading for High Street Ken. - Didn't know they existed at all nowadays
I thought the same but I was on one last week.
When I got on at Kings Cross the destination blind said Hammersmith, but between Baker Street and Edgware Road an announcement came over the intercom saying "This train's destination has now changed"
As most people on H&C westbound trains bail out at Paddington anyway it wouldn't have caused too much of a problem, although doubtless there would have been some off to Westbourne Park, Latimer Rd etc who suddenly found themselves having an unexpected change mid-journey
Edited to correct formatting and typos