A limited company makes an online service, hundreds of thousands of people use said service for a fee of let's say, $200 per year. This service is relied upon, and no alternatives exist. Can the company start charging premiums for additional benefits within the service to extract more money from it's client base? And can said company modify the service to a point where it is no longer unique and the relied benefits of the service are no longer acceptable to the older customers, leaving no alternative places to find the former benefits of said service, and any services attempting to recreate a similar service would be at risk of being sued by the company in question for using their service basis. In addition, can the company at any time change to a free service model and charge $50 per serious service usage and only maintain reasonable value on trivial usage of said service?

Thanks for any answers ahead of time, i am really curious.