Answer:
No he cannot.
Explanation:
A contract of employment is a legal binding that exists between an employee and his staff. None of these parties has no right to change the contract just on their own accord. If a change has to occur it must be on mutual grounds between the employer and employee. A one-sided variation is a breach of the contracts of employment and this makes it illegal.
Depending on the extent and impact on the job, the employer should meet with you and explain their case for making these changes.