I certainly hope they didn't do any of the "repertory company" kind of casting that was done in the original and 1991 series. In the original series, it was really essential, because they needed to keep their actors busy (and not hire new ones). In the 1991 series, since they included scenes from both 1790 and 1991 in each episode, it might have worked a little better to cast different actors in most of the parts, but it was fun to see the actors play two different characters.
With the movie being a one-off, I think it would only confuse audiences to see the same actors play different characters in the past and "present". Much better to just have an actor assigned to one role, unless there is a very specific story reason to have them play two roles, such as with a reincarnation.