The metre is the length of the path travelled by light[...] time interval of 1/299 792 458 of a second

Question

If this defines a metre, then how could the speed of light be calculated before this definition? Did the previous definition of the metre get tossed in order to make the speed of light a universal mathematical constant?

Answer 1

Yes, the previous definition of a meter was tossed. There have been many, many definitions of the meter (and other units) over the course of history. As more and more precision becomes obtainable, the definition must change to reflect that precision.

It was not tossed "in order to make the speed of light a universal mathematical constant." It had already been observed that c was a constant. The invariance of c has absolutely determinable observational consequences which are, in fact, observed on a daily basis.

Answer 2

The meter was previously defined without reference to the speed of light. It was, in fact based on a particular bar of metal, with markings for the beginning and end. "Historically, the metre was defined by the French Academy of Sciences as the length between two marks on a platinum-iridium bar, which was designed to represent 1⁄10,000,000 of the distance from the equator to the north pole through Paris." Given the then current distance, light traveled at that speed, so they then re-defined the meter using that speed when they wanted to add consistency and exactness to the definition.