If Mitch Ryan had played Jeremiah, the character would have been written and costumed differently. I can imagine him giving off a few wry looks conveying how strange he found it to be in love with Josette, whereas Anthony George tended to look confused by the situation. Also, Jeremiah’s complete lack of interest in Millicent would have been more interesting.
Additionally, Jeremiah would have lived longer, because Mitch Ryan would (I assume) have had no thought of leaving the show. I’ve mentioned before that I think that if Jeremiah had lived, there could have been some interesting interactions between Jeremiah, Josette, Barnabas, and Vicky, since Jeremiah had shown some interest in Vicky before Angelique had messed with his mind, and Barnabas in 1967 had shown himself to be capable of falling in love with Vicky.