To change the TC, they do not have to "get into" the tranny. They simply drop the tranny out of the truck, pull the TC out (its a slip fit), slip the new TC in, put the tranny back in the truck. Done.
Now, its the DETAILS they need to follow.
You MUST REPLACE the tranny cooler. You must also replace the stock IN-PAN filter. You should also ADD an external tranny filter such as a Magnafine. Install it on the OUTPUT of the tranny cooler.
Without those details, all the crap that may be floating in your tranny can just foul everything up again.
I would also look at doing a new valvebody, or at least the sonnax valves and tricumulator springs. This will increase line pressure and make the TC lockup better so it doesnt slip as much.