Generally, any changes in the terms of employment must be agreed by you, and such includes changes in Title, Salary, etc.
Usually, promotions and increments are one-sided, as employees are not expected to reject things that are beneficial to themselves. However, there are times when the purpose of increments are to ensure employees are not covered under Part IV of the Employment Act, where OT payments are legislated.
I believe a good practice is to have employees accept in writing for the proposed Promotion and Increment. Else, it can be deemed as illegal and void.