I accept your point, John and Jim, but sometimes such announcements are indeed made at the very last minute:
Last night I was on the (25 mins late) 15:37 from Ealing Broadway & it was only once we were actually in Reading Station waiting to pull out that they announced it would now be non-stop to Oxford. Result - mad panic & scramble to get off before we got whisked off to Oxford (I wonder if everyone made it - anyone who was listening to a personal stereo wouldn't have had a hope!) ... .
Occasionally, too, I've experienced on board train announcements being promptly contradicted by platform announcements at Bristol Temple Meads (bloody Digital Doris again!), causing no end of confusion!