I'm doing something similar. Split cowherd into two groups: one younger, smaller with good udders and calving ease. The second group of cows is older, bad udders, too large, etc.   First group has an excellent registered Hereford bull with them to make replacement cows as well as a keeper bull. Second group has a terminal Fleckvieh bull with all calves going to processor.  We'll see how this works out and start culling cows over the next few years. Cows will gradually move from the first group to the second group after their 2nd calf is weaned.

Ordinarily the older cow group would be the place to make replacements, however due to poor bull selection/lack of knowledge about EPDs on my part 10 years ago, my older cows are too large with poor udders. My younger cows are much better structurally and are the type I want to grow my cowherd.