I think someone already stated how much crud can be inside the tranny oil pan. I would not do a flush on any used tranny. The flush process can mix up the junk in the pan and plug up the important valves in the Techm and cause it to fail. If you want new fluid do it old school, drop the pan, clean it well and install a new filter before finishing up.
There is a company in Florida that rebuilds the Techm, I had one rebuilt, they programmed it to match my Escalade exactly, plug and play for well under $500 delivered. I do have a scan tool that helps on the install but I am not sure it is necessary with their programming. If you are pulling your pan off most of the work is done to install a new Techm.
A trick for those doing this job on the 2007 and newer trucks. If doing this on jack stands to support the vehicle. Take a small trucker ratchet strap, you can attach it to the exhaust and ground it to the bottom of your jack stand. Tighten it up slightly and you will have clearance to get the pan off with the exhaust pulled down by the strap. I have done this 3 times now, saves a lot of time pulling the pan on a 6L80 trans. Without doing this the pan will almost come out but you will get stuck. Some instruct you to remove the exhaust pipe which takes a lot of time. The ratchet Strap trick works. I have one that is only 1.5" and it does the job.