They can:
What if you and your employer don't agree?What your employer can do
If you don't agree, your employer is not allowed to just bring in a change. However, they can terminate your contract (by giving notice) and offer you a new one including the revised terms - effectively sacking you and taking you back on. Your employer would be expected to follow a statutory minimum dismissal procedure.
More info can be found
here.
Why is everyone so against it? I can't see what difference if makes and being paid on the same date every month would make it easier to set up DD's etc for payday!