No duty should be changed in an ideal world but a Contactable is just a duty subject to change like any other.
No, they can't call you off a contactable like a standby but they are well within their rights to change it ahead of the actual duty providing minimum rest is complied with.
https://www.chirp.co.uk/upload/docs/...B%20114%20.pdf
Page 6.
Before you shoot me down I like it as much as you, but thems the rules.