I would add another layer to the higher ceiling.
You don't want to do that.
It wont look right (I doubt) but you can try.
You will need to use a LOT of mud and feather the joint as wide as possible.
Ugh, no, dont even go there. just add some drywall and be done with it. It will be much faster and look the best. It may even be the cheapest route.
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