Change in Agreement

my original contract states i work 40 hours a week but each year i have had an agreement and expectation that i will work 16 hours a week from March to October as I am a University student and then full time over Summer. That means every March, i sign a contract stating my hours are decreasing, and every October, I sign a contract that my hours are increasing again. When I went back to full tim in October 2024, I had the same verbal agreement with my Manager that I would be returning back to part time in March. I have just had a meeting with my Manager who has informed me that after all the restructuring and redundancies that occurred in December, I likely will have to work a minimum of 24 hours a week otherwise they won't be able to accomodate the shift which I definitely won't be able to do with my Uni timetable. I just want to know what my options are? Would i have to decline the switch and them make me redundant or would I have to quit bc i can't work full time? Even if this has been an ongoing agreement since the beginning of my employment - thanks in advance :)