This altered mission statement is actually the "What We Do" section of the "Science and Technology" sub-office of the "Office of Water" within the EPA. The actual EPA mission statement is exactly identical to the December 2016 version.
The three-paragraph "What We Do" section of the Office of Water is also unchanged.
OW is responsible for implementing the Clean Water Act and Safe Drinking Water Act, and portions of the Coastal Zone Act Reauthorization Amendments of 1990, Resource Conservation and Recovery Act, Ocean Dumping Ban Act, Marine Protection, Research and Sanctuaries Act, Shore Protection Act, Marine Plastics Pollution Research and Control Act, London Dumping Convention, the International Convention for the Prevention of Pollution from Ships and several other statutes.
Headquartered in Washington, D.C., the Office of Water works with the ten EPA regional offices, other federal agencies, state and local governments, American Indian tribes, the regulated community, organized professional and interest groups, land owners and managers, and the public-at-large. OW provides guidance, specifies scientific methods and data collection requirements, performs oversight and facilitates communication among those involved. OW helps the states and American Indian tribes to build capacity, and water programs can be delegated to them for implementation.
The Office of Science and Technology is one of five offices under the Office of Water within the EPA. The other four have "What We Do" statements that remain unchanged.
The tweak to OST's What We Do statement, “probably the most important thing we’ve found so far,” according to the EDGI watchdog, reversed the order of the two sentences, introduced a grammatical error, and, yes, changed "sound, science-based standards" to "economically and technologically achievable performance standards."
I am reluctant to predict a major shift in EPA behavior based on this rewording. If this altered text is "the most important thing" then I think we should recognize that almost all the What We Do language, and the actual mission statement of the agency, with it's goal that "national efforts to reduce environmental risk are based on the best available scientific information" remains identical since the previous administration.