We fixed the speed of light by definition in 1972.
Already by 1960, J.L. Synge (Relativity, the General Theory Ch. III §2) taught:
"For us time [or rather, duration] is the only basic measure. Length (or distance [or, indeed, quasi-distances]), in so far as it is necessary or desirable to introduce it, is strictly a derived concept, and will be dealt with later in that spirit. "
In 1983, on the other hand, the 17th CGPM defined (effectively) the SI unit of "speed", i.e. the ratio of SI base units "m / s", as the $1 / 299792458$th of the speed of light (in vacuum).
However, the value of the speed of light (in vacuum) itself is of course unaffected by any particular definition of units.
He [the speed of light (in vacuum)] might still change,
That appears doubtful. As long as reference is made to the same definition of "ligth (in vacuum)", and as long as "length" is strictly understood as a derived measure (with the same derivation or definition used consistently),
the notion "speed of light (in vacuum)" plainly remains unchanged.
A note in consideration of the already published answer by Kyle Kanos:
Of course, the speed of light (in vacuum) being constant (by definition of "length", and thus of "speed") does not preclude the electro-magnetic coupling (referenced to vacuum) of some particular given charged particles to be found of different value, in different trials;
nor, for instance, the length of some particular given "platinum-iridium bar" to be found of different value, in different trials.
And how does official science explains really the famous drop in the measures of c?
My own assessment (which is hereby public and open for comments/responses):
It appears doubtful that people who claim to have "measured the speed of light (in vacuum)", (rather than, for instance, having measured distances between different identifiable parts of experimental equipment, or whether they were at rest to each other in the first place; or having measured the index of refraction in a particular experimental region) were able to assign any finite range of systematic uncertainty (or confidence intervals) to their reported results at all. Thus any possible such "drop" appears insignificant; and one may not strictly speak of such reports as "measures of c" in the first place.