Decisions to keep running these plants—which are often more than 40 years old—have sparked court battles in New York and are raising questions about how much should be done to retain legacy fossil-fuel facilities.

In the past four years, at least 29 coal units in 10 states have switched to natural gas or biomass, according to SNL Financial, a market data firm. Another 54 units, mostly in the U.S. Northeast and Midwest, are slated to be converted over the next nine years. The future and completed conversions represent more than 12,000 megawatts of power capacity, enough to power all the homes in New England for one year.

By switching to natural gas, plant operators can take advantage of a relatively cheap