Thanks for your reply. I should have been more mindful about providing enough information to answer my question.
The Italian Cypress trees in question are almost 20 years old, with the largest two trunks about 12" in diameter when measured 6" above grade.
The two most important trees are located in a wide spot in the rock-bed, so It is important that we are able to plant near the same location after the current trees are removed. Most Italian Cypress in DFW look stressed like ours, so we would like to replace them with something more capable of tolerating our hot summers, thus out choice of Crepe Myrtles. My large concern is how to remove enough of the Cypress stump and roots to allow planting of their replacements. I'm concerned that we may need to just leave the Cypress stumps cut-off at grade and covered with rocks, as the we've removed others and the Cypress stumps seem to last for decades without much decay.
Thanks for reading.