Can anyone help with some advice?

My sister is a single mum of a 9 year old. She took a new job, full time, as a senior assistant in a special school for kids with learning difficulties.

She had a 40 hour week contract with set shift pattern but asked a few weeks after joining to change the shift by 30 minutes so that she could accommodate her kid’s drop off and pick up etc. Her work agreed- but she had to drop to 37.5 hours a week.

Work have now asked her to revert to her original shift pattern (meaning a later start but still 37.5 hours). She was also given the option that she could stay on the current shift- but would lose her “senior” position and have a reduction in pay.

Can her work do that?

Thanks!
Liam Dolan