Since it is from a rootstock, here is what I would do:
1. This year remove half of the bottom limbs below the top of the post. Make clean cuts right up against the trunk.
2. Next year remove all remaining limbs below 5 feet high.
3. Keep these lower limbs trimmed off if they try to grow back.
4. In the third or fourth year I would topwork the tree to the desired cultivar. You could get into the scion exchange thread, and someone would likely send you some more Dolgo or maybe something else you would want.
Good luck.