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Basically assume that during the Great Crusade, one sudden morning Perturabo wakes up in Rogal Dorn's body with his consciousness transported to Dorn's body, and Dorn in Perturabo's.

Assume no one not even the Emperor himself can figure this out. How will this affect the course of the Great Crusade?

Iron Warrior Marine: So, you want us to build new fortifications and use cunning in our sieges rather than needlessly throwing away our lives in a war of attrition?

Rogal: Yes.

IW Marine: ...this feels so wrong...

Rogal: I know.

...

Imperial Fists Marine: So, you want us to needlessly throw away our lives in a war of attrition rather than build new fortifications and use cunning in our sieges?

Perturabo: Yes.

IF Marine: ...this feels so wrong...

Perturabo: I KNOW!

I think the main takeaway of the OP is what would've happened if Perty wasn't treated like garbage? Personally I think he would've been one of the most beneficial Primarchs to stay loyal compared to MANY of the other choices.

True, but probably not at the expense of Dorn. If you'd had both on the same side....