I think it's just the nature of the business. I might be wrong, but VAs have a contract-based hiring system, and their job pretty much ends when their deal is met. From there, they are either hired again for whatever reason (especially if the company thinks they are excellent), or they are layed off and get replaced for whatever reason.

What most likely happened is that the original cast had a deal with the only the original, vanilla game, and when Heavensword popped up some may have been unavailable/weren't rehired for whatever reason. Unless there is an official statement as to what happened, however, all we can do is just guess what went down.