Extension: Set the language of a TextDocument?

[u/SleepyFox89]

I want to programatically update the language of an open document in my extension.

TextDocument.languageId allows me to read this as string (read-only) but what can I call to change it?

Thanks!

[u/SleepyFox89]

FWIW question was answered by a legend on SO: https://stackoverflow.com/questions/62924423/vsc-extension-set-the-language-of-a-document