I have seen some posts regarding dealers using their Tech2 to check for reflashed ECM's when drive train warranty work is needed. I cannot find the posts that show what Chris has to say.
On another note, I have access to the Tech2 at my part-time place. I checked the ECM on my 2007 Vette which has been reflashed MANY times, and found there were differences that were mentioned in the GM bulletin regarding warranty questions. Bulletin 08-06-04-033C. I then reflashed the ECM back to the ORIGINAL file that was in the car when I bought it brand new. Checked with a Tech2 and now there are NO differences compared to the info that GM wants you to look for.
So, I hope that answers the question about installing the original flash if you want to. I am not condoning nor disapproving this practice; just merely stating the results I have seen. Also not saying that this works the same with ALL vehicles.