When and why did the bosses' voice acting change?

I have noticed that many Monolith bosses, including Lagon, Heorot, and Stolen Lance bird, etc, have had their voice acting change.

The lines themselves are the same, its just a different voice doing them. A much worse voice with a weak delivery and lacking the classic nature of the original voice acting. Why was this changed? What was the design rationale for re-doing something like this?