The issue is, how can someone new make this decision about already existing employees?
I don't understand the "how" form of this question.
New employees that he hires, sure. But existing ones?
Apparently so.
Is being re-background checked and re-drug tested legal and/or common?
I don't understand what you mean by "
re-background checked" and "
re-drug tested."
You told us that this is a new requirement for both new and existing employees.
Obviously, drug testing isn't a one-time thing. If ABC Company hires Bill and requires a drug test as a condition of being hired, that test only covers what's in Bill's system at the time of the test. Bill might test clean and then go on a bender to celebrate his new job such that he might test positive two days later.
Likewise, a background check only covers what exists at the time of the check.
What I think you are intending to ask is whether or not it's legal for an employer to change the terms and conditions of employment for existing employees (whether it's "common" isn't legally relevant). The general answer is yes. Most employment in the United States is "at will," which means the employee can terminate the relationship at any time and for any reason, and the employer can terminate the relationship at any time for any reason that is not expressly illegal (e.g., discrimination based on race, ethnicity, gender, etc.). The most common exception to this is for union employees. The relationship between a union employer and union member employees is governed by the collective bargaining agreement. Other exceptions are for civil service employees and those who have a formal, written contract that limits the employer's ability to terminate the employee. Part and parcel of "at will" employment is that the employer has almost unlimited ability to change terms and conditions of employment.
Things like background checks and drug testing are sometimes governed by specific state (and federal) laws. I don't know if any such laws are applicable in Alabama. However, in general, there's nothing illegal about an employer imposing new requirements on existing employees.