There are several factors that should be taken into consideration when thinking about replacing a water main in NYC. The 3 main factors are:
It takes a high level of knowledge and skill to research and conclude upon what the best decision is for each scenario. The average “plumber” may not have the skill set or experience to provide the necessary information needed to make an educated decision. You should always consult with a licensed water main and sewer contractor who specializes in replacing water and sewer lines on the outside of the building.

The cost of making two water main repairs in a lifetime is much greater than the cost of replacing the entire pipe all at one time. It usually makes sense for a homeowner to replace the entire water line instead of risking the possibility of making a second costly repair in most cases. There are scenarios when a repair may make sense on a newer copper or ductile iron water line.

If you water line is in the range of 25-35 years of age you should factor your future plans with the house to assist in making an educated decision:
The potential of rising cost factors should be taken into consideration as well. If your water main replacement cost $3,500 today, will it cost you $4,500 in 5 years? The overall cost of replacing a water main has increased over the previous 5 years due to the increase in material cost, DOT restoration requirements etc.
If you currently have a lead or galvanized water main it is 100% illegal to make a repair as per NYC DEP plumbing code. Both of these materials corrode and become fragile over time making it a less than plausible decision. The corrosion on the inside of the pipe will decrease the water flow over time, potentially impacting the volume and pressure of water entering your home.
It is legal to make a repair on a copper, ductile iron and corporation pipe in NYC and it may make sense in some scenarios however, you should consider all of the factors mentioned in this article to assist in making a wise plumbing decision.