I would hope that the original actors would be willing to recreate their characters, and I'm not sure just how much actual time would be required of them to make a film in this format. The original Star Trek cast reprized their roles in an animated series for Saturday mornings a few years after the regular show's cancellation, and they were able to do their voice overs for an entire episode in a single day. That was a completely animated show, of course, not live action, and I'm not sure just how much more live action was required to make 300 vs a regular live action movie. If any of the original cast either wouldn't or couldn't return, the character could still be used via computer generation, and wouldn't be any different than the video game "Chaos Bleeds," where Sarah wasn't available, so another actress furnished the voice. Though I'm sure that we would all prefer the original cast, with computer generated action, it wouldn't be really necessary, unless there were some legal problems involving using their images without paying for them.