Hello, I bought a 2004 Chevy 2500hd with 200,000 miles. Snow plow work truck. The transmission fluid is dark and the transmission hesitates to shift 1st and 2nd gear. Should I drain the old fluid first and refill with new fluid along with Trans Tune to act as “flush”? Then drive a couple thousand mile and then drain and refill Trans Tune as a “conditioner”? Thank you!