if they have already made changes to others by making certain positions redundant and creating new ones (which could be fairly similar in task, but with different hours, or at a different location) for those facing redundancy to apply for, i would think they would probably do the same thing again if they wanted to change your post, as thats an accepted fair way to implement those kind of changes
one thing to bear in mind however is that altho your contract may say your working hours can be changed to suit working needs, if you have been working there for some time and your hours have always been the same and never changed, or they haven't changed in a long time, those current hours could be considered as your standard contractual hours in implied terms, so whilst the paper says one thing, you do another, and it's what you work by which can count. for example, you would get one contract when you start, and it may say your pay rate is £15k pa, but you now get £19k pa even tho your contract doest reflect this
if your hours already vary, say sometimes you do 8-4pm and others 10-6pm then it might not be as unreasonable to change to another set of hours as you are used to them changing regularly which is in line with the contract, but if they were to change the hours so you are working at a time much earlier or later than you would normally do, such as 10pm finish, then that may be unreasonable
to cut a long story short, they can change the departments hours if they want to, and change anything else, but they should be following certain procedures, so they could just make your current post redundant as they no longer require staff to work fixed hours and instead have an opening for new posts with flexible hours, and if you don't like the new hours and they are going for it, you have to make the choice to leave/be made redundant, or put up with the change