I tend to doubt that Gerringer would have played Jeremiah. I've never seen so much as a hint of that in any sort of DS publication or article. And, as mscbryk correctly points out, they were paving the way for Anthony George to play Jeremiah (as they did with Nancy Barrett to play Millicent, Joan Bennett to play Naomi, and Louis Edmonds to play Joshua) long before Gerringer was replaced. To me the story seems more like one of those rumors that somehow gets started and then gets spread - like the notion that Joel Crothers revealed in a Soap Opera Digest interview that he was supposed to have played Quentin, when in fact all Crothers actually said was that he would have liked to have played a character like Quentin because Quentin was far more complex than Joe had been.
Also, it's funny how we sometimes perceive things. Gerringer was actually 5 years younger than George (Gerringer was born in 1926, while George was born in 1921), so, even in the unlikely event that Gerringer might have played Jeremiah, it wouldn't have necessitated any sort of change at all in the way we saw the Jeremiah/Barnabas relationship play out in 1795/96. In fact, Gerringer would have been closer to Frid's age (Frid was born in 1924).