that was a really insensitive comment

Maybe the boss isn't legally obligated to, but it is common curtesy, and being an employer doesn't automatically give you the right to be a ******* rude asshole, let alone apparently verbally (or possibly even physically or sexually) abusive to the point of making an employee cry because they wanted to know why you changed their shift without telling them. A boss is supposed to lead by example, not sit with their thumb up their ass, treating employee like toys they can mess with, break, throw away and replace to entertain themselves or make them feel better about them self. If you are an employer you are just as ******* responsible for being a decent human being as every other person is. And if the boss didn't notify OP about the shift change, yeah, that could very well be a legal issues, or if the boss took away vacation days/days off they had agreed to give OP earlier. Also, the world being shit isn't a reason to add MORE shit to the pile.

Where I'm from they need to give you legally 24 hours notice before changing a shift..

Your boss should still be human! And yes, if you have already been scheduled in (as it is not a brand new rota), of course they need to consult whether or not you're free that day! You were given a schedule, you made plans accordingly so they wouldn't clash with your job. If that schedule is now changed, you can't expect the employee to be available anymore.