For example, the speed of light was defined to be exactly 299,792,458 m/s. Now imagine we develop some apparatus that measures it to be 299,792,458.6 m/s, with all experimental error accounted for, etc. How would this effect the definition of the meter, which is based upon the speed of light?
See the definition of the metre on Wikipedia:
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458 second.
So we'd change our definition of the metre to:
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458.6 seconds.
I think the important thing to note here is that the speed of light is constant by definition. It's defined to be constant. See what John Moffat and João Magueijo said in their 2007 Comments on “Note on varying speed of light theories”
“Can c vary? Could such a variation be measured? As correctly pointed out by Ellis, within the current protocol for measuring time and space the answer is no. The unit of time is defined by an oscillating system or the frequency of an atomic transition, and the unit of space is defined in terms of the distance travelled by light in the unit of time. We therefore have a situation akin to saying that the speed of light is “one light-year per year”, i.e. its constancy has become a tautology or a definition”.
And it isn't constant. It varies in the room you're in. That's why optical clocks go slower when they're lower. Einstein said this in 1920:
“Second, this consequence shows that the law of the constancy of the speed of light no longer holds, according to the general theory of relativity, in spaces that have gravitational fields. As a simple geometric consideration shows, the curvature of light rays occurs only in spaces where the speed of light is spatially variable”.
See my "physics detective" essay the speed of light is not constant for details and references.