Worth noting that continued employment is considered sufficient consideration for many such contract changes, in "at will" employment states.

This is simply not true. It has nothing to do with at-will employment. It is state law. Some states require consideration separate from salary, some say continued employment is sufficient consideration. 49 out of 50 states recognize the doctrine of at-will employment. Only Montana uses just-cause instead (just like the rest of the developed world coincidentally).