For those who like to see for themselves, the before and after versions are available.

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.

The Office of Water (OW) ensures drinking water is safe, and restores and maintains oceans, watersheds, and their aquatic ecosystems to protect human health, support economic and recreational activities, and provide healthy habitat for fish, plants and wildlife.

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."

before

OST is responsible for developing sound, science-based standards, criteria, health advisories, test methods and guidelines under the Clean Water Act and the Safe Drinking Water Act. We work with partners and stakeholders to develop the scientific and technological foundations to achieve clean water through national programs that protect people and the aquatic environment.

after

OST works with states, tribes, and other stakeholders to develop recommended safe water quality levels for toxics, nutrients, and pathogens to help ensure our nation's waters can be used for fishing, swimming, and drinking water. OST also develops national economically and technologically achievable performance standards to address water pollution from industry.

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.