I have the same question, new to me, 2011 Chevy 2500HD. Likely due for a transmission flush. Unsure if it ever been done, I'll have to go through the service records that came with the truck. Sounds like the Transynd is the way to go. But if there's Dexos in it can they be mixed? If I decide to go with the Transynd?
They will mix fine. No flush required (and not Allison recommend either). Change it, drive it for a bit, change it again. Allison considers that good enough for extended warranty in applications they provide warranty (non pickup application). If you want to get super picky, look up the Hoot method, not required but it is more complete (and more work, fluid). When I switched over I just did the double drain, that was ~200k ago in 2008.