I have a 60 amp 208 single phase circuit using 4 AGW Aluminum wire 75-100 feet away from panel. I need a 30 amp 208 single phase circuit for an expresso machine demo. Can I just change out the 60 amp breaker with a 30 and use for the day or will the larger wire size not be safe.
Having larger wire is ok by changing to a smaller breaker you are protecting the coffee maker, this is fine in code just make sure the new outlet is rated for aluminum wire.
The most likely problem will be in connecting the larger conductors to the devices designed for smaller ones. You may have to use a "pigtail" arrangement, but that means lots of room in the box.