How do I get my 666 house number changed when neighbors will not agree to change theirs?

Go to a city council meeting and see if they can help you. Otherwise, if the ordinance says your neighbor has to agree, there usually is no legal mechanism by which you can force them. This is usually the domain of the city government, which is represented by the city council. These people are elected officials. Put pressure on them--they may not want to be associated with forcing that unfortunate number on someone when they run for re-election.

Several agencies have to agree to change an address. This is a transaction between the municipality, the emergency management agencies and the United States Post Office - and maybe others (utilities may get involved). No municipal government is going to petition the other agencies to change an address if there is conflict. Every change of address I've been involved in was done in complete agreement. You probably have no right to try to force a neighbor to agree to change their address. As much as you may not like yours, maybe they picked their house because the street address was their lucky number. Who knows? Sometimes you need to offer consideration to get them to change their minds. Consideration in legal terms usually means "something of value." Cash is the most common consideration. Maybe you buy them a new house number? Don't have any idea what the going rate is for someone to change their address. Depends on what you and the neighbor believe is fair. Good luck.