Just to give you an answer that is outside what could be described only charitably as outside my area of expertise I have heard that American English has remained more constant as to what it is than English English. It is likely that the difference was marginal at the time but has since diverged significantly. It is not likely that you would identify the accents of the founding fathers as English. By which I assume you mean the old BBC English. Here is a link to a reddit comment that seems credible to em on the subject.