Hi Double Boast crew,
I am unclear why you would want to remove the round bars, such as The Yakima System uses, to install a different set of crossbars. The round bars integrated on our Ranger Tugs support various kayak mounts, so the Thule crossbars are not necessary. If you have the Thule crossbars on your vehicle, and want to use the same kayak saddles or other kayak "holders" that are from Thule, I believe Thule sells adapters so those can fit the round bars.
A quick phone call to a specialty rack shop or REI rack dept. will clarify integrating components if by chance that is an issue.
If you are indeed wanting the complete Thule system including crossbars, perhaps a fabricator would be a route to explore.