Was the pronunciation of the name Kahless changed deliberately for Discovery?

It was not changed deliberately, but rather through a series of what might be called accidents. In some ways, this change resembles the evolution of how words are pronounced in natural languages, and in other ways, it's quite different.

When Marc Okrand decided on the sounds of the Klingon language, he had only the sounds used in Star Trek: The Motion Picture to go by. He also went back and watched the episodes of TOS with Klingons in them. He decided that Klingon names were really pronounced a certain way natively, but that what we heard was a sort of Anglicised version of them. So, for example, "Krell" was really Qel (where the Q is what linguists call the voiceless uvular affricate).

The name "Kahless" was spoken only in the TOS episode The Savage Curtain, and Okrand decided that the closest pronunciation in his language was qeylIS. Since he didn't expect the name to be spoken very often again, he wasn't going for consistency. However, TNG expanded on the story of Kahless greatly. The writers and actors on TNG didn't take particular care to be consistent with TOS, and in fact, the name was pronounced in several different ways before it settled on "kay-less". This is the pronunciation which stuck.

Star Trek: Discovery took much better care to be consistent with Okrand's Klingon. The actors actually received voice coaching, so that even when Klingon characters spoke English, they spoke with an accent as if English was not their native language. As a consequence, the name of Kahless sounds the way that it does.

This one video may give a general taste of how the Klingon in Discovery returns to Okrand's design: