If a Special anime was made by a different company it might be more likely to use different voice actors for the characters as well. To be honest a lot of factors affect who the voice actors would be.
As for me personally, I would prefer different voice actors for all (or at least most) of the characters, since they are all pretty different from their anime counterparts for the most part and I'd like to create distance between the series. However, if the anime voice actors actually did a good job I suppose I wouldn't mind some of the anime voice actors returning. To be honest I'd really just need to see what kind of voice the anime voice actors would do for the characters before deciding if I'd be okay with it or not though. XD
What about manga-only characters with no anime counterpart though? :3 Who would voice Blue, Yellow and Emerald?
"But sometimes I have to ask myself this question. It's true that to us his imaginings are nothing but the inventions of a busy mind. But to him, there simply is no other reality. Furthermore he is happy there.
So why, I ask myself, why in the name of healing him must we drag him painfully into the world of our own reality?"
~Doctor's memo, from Silent Hill 2