If someone can find a link to this law please post it. If true it would have to be a Las Vegas city ordinance or as stated above a Clark county ordinance. I did find an article where Nevada changed the residency law from 60 to 30 days
See Here.
Notice where it specifically says
"But the March 21 opinion by Legislative Counsel Brenda Erdoes said that border state residents, tourists or seasonal residents in Nevada do not have to change plates even if they are in the state for more than 30 days in a year. Requiring them to do so would place an "unconstitutional burden on the ability of the nonresident to engage in interstate travel,"
They also state that this is a secondary offense and you will not be stopped for having out of state plates.