A decision to pay shift allowances is a company policy one. It is not part of central Government legislation. You need to refer to your company's Handbook if there is one, or previous norms. Unless its part of policy, you have no entitlement.
If your offer of employment had a clause in it stating your hours of work, you may have a case to decline to change the hours. However most companies merely state the number of hours, not the shift pattern, or put some clause in to say they may change it according to future business need.