What @kritiper wrote. It would not be a good idea to change the size of a meter, meaning all the endless things we measure in meters and so on, would be inaccurate, and if you wanted more than 1/10% precision on any measurement (which you would, at least for some engineering tasks and other precise matters, as well as practically every scientific book and reference written in the last 100 years), then you would need to make sure whether you were referring to a source before or after the change.
Spacecraft would probably end up crashing because occasionally someone would get some number wrong. Etc.
The speed of light is difficult to measure with accuracy and is one of the least-used figures there is, and the people who use it are also since 1960 well-used to it being 299,792,462 ± 18 meters per second, so they wouldn’t thank you either.
For the history of what the meter was defined as over the decades, and why, see for example this rather good NIST article.