No, an increase is unlikely. The rate may increase as a result of a change in law, not because energy prices are spiking. For example, let's say that Massachusetts passes a law that says "all energy suppliers must pay a 50¢ per kWh tax." Then the rate can change - the change in the law creates a direct, material increase in their costs - literally raising their costs by 50¢ per kWh. Likewise, maybe the state says, "the amount of green energy must go from 50% to 75%." That isn't quite as direct, but certainly direct enough.

They can't just raise rates because they want to in the middle of an agreement. However, no business would agree to a contract where the legislature could pile on taxes, fees, and all sorts of regulatory requirements while committing to a fixed price.

Also, the rates do change when the town renegotiates the rate at the end of the contract. The contracts are usually a couple years. If your town negotiated the rate in 2021, you got lucky and are probably locked in at 10-11¢. If your town had to negotiate this fall, it's probably 15¢.