If you're quick and can change plugs when the engine is hot, the holes are "larger" because aluminum expands at a higher rate than the steel spark plugs.
This makes the holes slightly larger than when the engine is not fully warmed to 190 degrees.
If they turn initially and start to get tight, spray some penetrant/lubricant down the hole and call the customer to explain the car will take another day.
Then you carefully unscrew them with a swivel spark plug socket to reduce any side loading which will cause them to bind further.
Of course, you can't get the engine hot the next day but the penetrant should make things easier.