You mean Raintree County, don't you Kitty? Probably confusing it with The Petrified Forest! I wasn't aware of that story but Elizabeth Taylor also replaced Vivien Leigh in Elephant Walk. The latter's scenes where she was filmed in long-shot were left in and this seems to be a common occurance. As mentioned only a couple of days ago when I paid tribute to Valerie White, who was replaced by Jean Kent in The Wicked Lady.
Another notable substitution was in
Billy Liar, when Julie Christie got her 'big break' at the expense
of Topsy Jane (seen here in
The Loneliness of the Long Distance Runner) who fell ill during filming.

Other 'Actors replaced' include Ann Holloway, the original Georgina Jones in Adam Adamant Lives before the part was recast with Juliet Harmer, and Judy Huxtable, whose role as Laura Slade in the Randall and Hopkirk (Deceased) episode For the Girl Who Has Everything went to Carol Cleveland ... who wore the same dress in The Baron I noticed!

"But that's another story ..."
Sorry, DBF, I didn't realise you pointed out the same mistake.