Generally speaking, an employer can’t make considerable and unwanted changes to an employee’s job or the terms of their employment. The types of changes that are prohibited are:

• A cut in pay, salary, commission or bonus
• Drastically reducing an employee’s hours of work
• Demoting an employee, or changing their job title or duties
• Creating a toxic or unsafe work environment
• Changing work locations
• Putting someone on a temporary layoff

Change in pay, specifically decreasing pay, could put you at risk of constructive dismissal. 
To answer your question, yes, it is best practice and legally sound to get all changes to the employment agreement in writing and signed by the employee(s) it affects. Keep those records with the original employment agreement.