Can I change an exempt salaried employee to an hourly employee in PA?

There is nothing criminal about this, ( you posted it in criminal law)

As long as there is no contract, no union you can change someone to hourly, you can change pay rates, take or add benefits or more.

If the employee is working a lot of hours, often you will find yourself paying more.
This year is the first year in 25 years I have ever been hourly, and I love it, I actually get more pay when I work over 40 hours, and I can't be asked to do work things on my days off.