Lots of blending in those groups, and differences even among those who are referred to as "Regular Baptists." If you're referring to those descended from the New Salem association in Kentucky, I'd say they were influenced by the mergers they experienced that led to their being known as United Baptists. The Sovereign Grace Association also considers themselves to be "Old Regular Baptists" but less influenced by other groups.
I think that the nature of being a Baptist church, and congregational polity, leads to a fellowship among believers within individual congregations or groups that lends itself to discussion and change, depending on the open mindedness of a congregation's members. The only experience I've had with Regular Baptists, or Primitive Baptists, was years ago in Eastern Kentucky and Southern West Virginia, the "Old Ragalars" as they called themselves. They were neither open minded, nor tolerant of any view that wasn't already embedded in their own presuppositions. They also believed that only the King James Version was "the preserved Word of God in English," which may also be something that affects interpretation. I was back down in that area last summer, and I noticed that a lot of the Old Regular and Primitive Baptist church buildings are abandoned, and the congregations long since disbanded and gone.