It is a nice problem to have. Unless they under a contract that specifies their hours, you only have to give reasonable notice. The more notice you give them the easier the transition. As you make the change you might consider letting the employees know that you will consider hardships in scheduling and might move some of them back to the original hours as more employees are hired and trained.
The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change.
Please see my answer to your similar question.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.