Is it legal for an employer to change a pay date without prior written notice?

The applicable statute is silent has to the changing of an established pay date. 

The law reads in part, "All employers operating a business shall establish regular pay periods and rates of pay for employees except executive personnel.  All such employers shall pay salaried employees at least once each month and employees paid on an hourly rate at least once every two weeks or twice in each month..."  Virginia Payment of Wage Law, § 40.1-29 of the Code of Virginia.

As for your being terminated, most employment relationships are what is known as "at will".  This means that you can work for an employer, or not, your choice.  In turn, the employer can hire or fire someone for any reason or no reason whatsoever.

The only exceptions to this are: 1) If there is an employment agreement or contract (which governs termination); 2) If there is a union or collective bargaining agreement (which governs termination); 3) If there is an employee handbook or other official policy statement (which governs termination); 4) If discrimination is a factor (there can be no termination for reasons of race, religion, age, disability, sex, national origin).