If somebody booked you to babysit from 7 and then sent an email at 615 saying can we make that 730? would you still expect to get paid from 7?
I suspect that they now expect either to pay me less or get back later, which I don't really think is on at this short notice. They were late back last time and didn't offer any extra so I am already not best pleased, but I don't want to jump to conclusions about what they intend and I'd be interested to know if moving the goalposts is par for the course.