It really depends on the team that make the TV series. If they want the TV series to stand on its own and free from the shadows of its source materials (or go on different direction), the director/showrunners usually will do seiyuu audition again to find the best talents that fit his/her vision of the anime TV series (Arpeggio of Blue Steel comes to mind which was a good decision). Whether or not previous game/drama CD/OVA seiyuus will be brought back depends on whether they managed to ace the audition or not.
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