Even though changing company practices would have a greater effect on the environment than changing individual diets, that doesn't mean that cutting down on individual meat consumption has no effect on the future of our environment. Financial support, in large amounts, puts pressure on corporations to change their practices. Even if someone wants to continue to eat meat, they can buy their meat from more sustainable sources than factory farms.
Cutting down on factory farm meat consumption in particular puts pressure on companies to move away from their most environmentally harmful practices. Factory farms make massive amounts of pollution. Family farms and other small scale meat productions don't even come close to factory farms when it comes to harming the environment. In particular, sustainable agriculture, practiced by many family farms and other small scale meat farms, is much safer for the environment. Sustainable agriculture recognizes the importance of raising only the amount of livestock that an area of land can handle. For instance, a sustainable farm will only raise a certain amount of livestock so that the amount of manure is not overwhelming. Factory farms are terrible at handling the storage and disposal of manure, leading to it releasing noxious, harmful odors in storage and contaminating water sources. The sustainable farm uses all of the manure that is produced as fertilizer for the farm's crops. Thus, the manure is constantly being recycled in a completely environmentally friendly way. Financially supporting sustainable agriculture would allow someone to continue to eat meat while reducing their individual contributions to climate change.
Organizations like Beyond Factory Farming and Farm Aid explain exactly how this is possible.