Being that the main reason they created 1795 characters for the already established cast of the original DS was to preserve their jobs over the many months it took to tell the 1795 story, I would prefer to see different actors playing the ancestors in the new film (if they start the film in present day and then travel back to the past, per Dom's initial question). Having new actors for the characters in the past wouldn't put any of the modern day actors out of a job (although it does suggest the need for a bigger budget to accommodate twice the number of cast members).
My reasons are: I got annoyed with Vicki (both in the original and 1991 versions) mistaking an ancestor for a modern day member of the family every time she met someone for the first time in 1795/90. "You look like someone I once knew." We, as an established audience, already love our show and can easily forgive this. But if you think about it, how believable is it for 20 or more people in a family separated by nearly 200 years to look exactly alike?
Also, the zinger was supposed to be Barnabas' resemblance to his "ancestor's" portrait (and also Maggie's to Josette's). If Elizabeth looked just like Naomi, Roger like Joshua, David to Jamison, etc... to me that greatly lessens the importance of Barnabas standing by his portrait and saying "Oh, Madam. You may tell her that it's Barnabas Collins."
And although I loved the 1991 version I found it hard to set aside my disbelief for Vicki looking exactly like Josette and even harder to believe the 1790 characters could accept it so easily. And after Josette dies and the hatred of her Aunt Natalie (and others) is directed at (the wrongly accused) Vicki, to me that hatred for someone who looks exactly like your recently deceased beloved niece just doesn't ring true. (Perhaps it would have been easier for me if the writers had Natalie, Andre and others say to Vicki something like "You have used your witchcraft to mask your own face with the face of my innocent Josette!")
And lastly, on the original show (and to a lesser extent, the 1991 version) it was never explained why a non-family member in the past looked like a family member (or other character) in modern day. (Natalie looks like Julia; Forbes looks like Joe Haskell, Noah looks like Harry Johnson, etc..) While we fans have had more than 40 years of fun trying to figure out this contorted family tree, why bother bringing up these questions if the budget allows for a different actor for each part?
SDP