"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.

I imagine it was an issue to some extent when it happened, but DS9 and ENT established they look different in-universe and it wasn't just a new design.

> So why is the change between TOS and TNG ok but the change for Discovery isn’t?

Because of the intention. The TOS Klingons were always supposed to look like the TMP/TNG Klingons, but the producers didn't have the budget until the movie and next show.

It's an entirely different thing when the original designers correct a budget constraint vs. new designers arrogantly throwing away prior art.

They cleared up the TOS Klingons discrepancy in an episode of Star Trek: Enterprise (Season 4 episode 16, “Divergence”).

Because TOS and TNG were awesome and canon needed a patch. Discovery sucked and got flushed.