I wonder if you could get away with Ben still wearing his “Kylo Ren” robes, but you make them gray instead of black. I feel like you could accomplish that with some color grading and masking.

Or, maybe you could use CGI, or film a body double wearing a big poncho, sort of like what Luke briefly wears in ANH, essentially covering Ben’s entire body. You could just put his head on top, if you could replicate his movements well enough. Even better if he’s just standing in the shot, and not walking.

The way I envision it, I think you would only need to use VFX in one shot to show that he’s wearing a new outfit. The rest of his shots would just be him in silhouette. I’ll share those images.