I went part time last year, mainly due to health issues, and it was one of the best decisions I have ever made. It has allowed me to cope with, and enjoy, the 4 days a week I do work now. But I really worry that at some point in the future management will say they want me to return to full time.
It is no easier for them to change your hours than for any other employee. It's not out of the question that they might try, but that would be a change to your terms and conditions, which requires your consent. While there are ways of forcing it through if there is a good business reason to do so, it's not easy and they'd need to consult with you.