What if they switched lightsabers at some point? There’s actually a few shots during the table fight where Obi-wan is holding Anakin’s lightsaber, but then there’s a shot in between where he’s holding his own so right now it’s a continuity goof. You could re-edit that section of the fight so that they lose their lightsabers, and show Obi-wan Force-calling Anakin’s saber to him. The next you see of them they’re fighting on the bridge where it’s too hard to see which lightsaber each one has. Then it’s just a matter of cropping a few shots or re-editing.

Sure, it still ends with Obi-wan’s lightsaber going over the edge with Anakin, but there’s enough of a difference in sabers between ROTS and ANH that you could chalk it up to Obi-wan making a new one.

Finally, you could include part of the shot right after Obi-wan picks up Anakin’s lightsaber, as he’s walking up the hill and starts to turn around. Then cut to a reaction of the lava streaming by, perhaps part of the shot before he says ‘you were my brother’, then the final shot of him walking away, though you’d have to crop it or paint out Anakin’s flaming body.