This also depends on the flavor of FORTH you're using. The one I wrote in high school (oh so many years ago) compiled directly to executable code, with constants compiled to direct pushes, so "redefining" a constant this way would run more slowly.

While I absolutely hate FORTH as a production language (I've seen millions of dollars flushed down the tubes by FORTH afficianados who were unwilling to admit that their code was unmaintainable, slow, and didn't work), it's fun to play around with. Everybody should write at least one FORTH in their career.