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What is the name of your state? Missouri
If you hire employees for part time positions and then increase their hours to full time when business increases, can you reduce their hours back to part time when business slows down? (Eventually they will be working full time again when business picks up in the Spring.)

Of course you can. Determining work schedules and changing them as is appropriate is entirely the pervue of management. You don't have to pay people to stand around doing nothing when work gets slow.

FYI - if you are the employer and you offer any company benefits (particularly group health and retirement), you'll want to check your Plan Documents to make sure you're complying with the stated eligibility requirements for employee participation.