That's normal in my book (over 5 companies worked for). My current mob regularly moves standbys (and reports) back and forth by 6 hours at a time after roster is published, often with 24 hours to go, without asking and expecting you to just say yes. All in the name of "you love the company don't you?". Try to say anything and they'll remind you, this is what you (and a thousand other complete mugs) signed up to:
1.7. The Client reserves the right to change any duty in a published roster where it is
considered necessary due to operational needs.