Can an employer move you to a lateral position, but decrease your salary?

Yes. Pennsylvania is an at-will employment state. This generally means that you are offered limited protections at your job. Your employer typically will have no obligation to keep you in a certain position or at a certain pay level. If the employer's motivation is purely economical, you most likely will not have a claim. Howver, if the employer's action was prompted by some other reason (such as your race or gender) you may have some recourse.

Disclaimer:
While I am licenced to practice law in your state, I am NOT your attorney, the above is NOT legal advice, and an attorney-client relationship has NOT been created.

Unless you had a contract for this particular position at that particular pay, they can reassign you and reduce your pay. Unless you are getting less than minimum wage, they can reduce your pay legally.

Contact a local employment attorney for review of your circumstances.

DISCLAIMER—This answer is for informational purposes only and discusses general legal principles, trends, and considerations and is not intended as specific legal advice regarding your question. This answer does not establish an attorney client relationship.