"So why is the change between TOS and TNG ok but the change for Discovery isn’t?" Because the changes to the Klingons are too trivial to be canon or not canon.

"So why is the change between TOS and TNG ok but the change for Discovery isn’t?" Because the changes to the Klingons are too trivial to be canon or not canon. It's mere makup not writing. While the changes in Discovery are not mere makup.