I like the cut, but I’m agreeing with John William’s Imperial March not having the right tone for the scene. I think a lower, slowed mix of the song would work well. We’re seeing the scene through Obi-Wan’s perspective, so we have the external horror of Vader, the monster, in addition to the internal horror of Obi-Wan seeing for the first time how far his former apprentice and old friend has fallen. The OG version of the song doesn’t quite give that, but a variation could. Something along the lines of this would work nicely(Starting about a minute in, after the initial fanfare so we get the low build-up as he enters the scene): https://soundcloud.com/slowverbtunes/imperial-march-slowed-reverb

Regardless, I’m liking the direction you’re taking, and looking forward to seeing how this goes!

EDIT: This was written with that first example in mind, the piano cut works really well!