I have a JBP, a JWP, and 2 hinokis. I have partially removed about half of the original soil last year on all of them. They’re responding nicely. I want to get all of the original soil gone and placed in an appropriate media ASAP. Assuming they all continue to look okay, could I replace the rest of the native soil with Akadama, etc, next spring so I don’t have to worry about watering two different types of soil or, should I wait 2-3 years before removal of the last native root ball soil? I want to get them into 100% media that I can depend on for moisture content, instead of trying to balance watering of half native soil and half decent media.
Yes you can, the key as you suggested is the response of the tree! If they respond with strong healthy growth the first year than the remainder of the repot is safe! If the tree appears weak or stalls with very little new growth in the first year it would be wise to wait.