Let Me Know if You Have Allison Transmission Fluid Questions

Hello,
I have a 2007 Duramax Chevy with Allison 6 Speed Trans. Had the Allison dealer change the spin on filter and refill with Transend. They say no need to drain everything. That's is only needed if you rebuild. The Chevy dealer say drain everything including the lines to the radiator and refill and replace filter. Who is correct? I took the cheaper route from the allison dealer.