Transmission went out after Body COntrol Module replaced.

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My car took several tries to get it started. Took it to have battery tested. Battery was fine, didn't receive bad gas everything looks good, then they couldn't get car started. Determined it was the BCM because they couldn't get it to reset. My mechanic (very trustworthy) towed it to a GMC dealer who replaced the BCM. They say car starts but wont move/no power and the transmission needs replacing, the trans fluid is very dark and a burnt smell. Car is only at 82500 and it isnt recommended til 100,000 miles. Had it towed back to house but tow truck driver drove it around the block and said he didn't feel/see any problems. Would it be worth having my mechanic change or flush the trans fluid? Was the dealer just screwing with me?