Hi!
I'm interested by Azalea only since a few months now but I now realize that this type of tree is a whole new world in itself.
There is a lot to know!
I have never tried to switch from potting soil to kanuma but from what I know I would really listen to the advises from more experienced members.
Here some info's that might be useful for you (Please take theses with a grain of salt... I don't have many years of experience so please correct me if I'm in the wrong!)
As
@Deep Sea Diver suggest:
If your plan is to move your azalea out of nursery soil and into bonsai soil, I’d recommend carefully root washing out all of the old media, including chunks of bark, perlite etc.
In theory I know you can do this if the tree was in ground and in good health.
If I wanted to do so from a nursery pot I would be even more careful if the health of the tree is not 100% so please judge your timing accordingly.
The ''sheen'' under the trunk of Azalea is a crucial part.
As you plan to work on that part of the root ball, I would ask if a ''half bare root is an option?
Whatever the answer make sure to really fill that part of the root ball with substrate, no air pockets as your tree apex could be weakened and possibly die.
I don't think would will find deep roots in the nursery pot as Azalea create very fine surfaces roots like ''hair'' size roots, and as
@Glaucus said earlier:
They are likely to be pot bound, with the roots forming a layer against the plastic pot.
So you will have to clean the top of the root ball for sure.
You have to make sure water can penetrate under the trunk so the ''sheen'' have access to water when the tree is watered.
If you plan on keeping the Azalea outside under the sun and the outside temp. is high, I would suggest a light color for the pot...Duh.. but depending on your climate I would also suggest to not go with a very shallow pot...
Azalea have a fragile root system... If the sun is out, the water demand will be high and if the roots are cooking in the pot due to the sun hitting it it wont be a very good afternoon for this poor tree!
Going with a slightly deeper pot will retain more moisture ''duh'' but will help cooling the pot.
If you decide to go with the repot, I would let the tree grow but vegetative grow only not wasting energy on flower production.
You will see Azalea can add meat on branches in a matter of weeks, they will back bud from ''everywhere'' if they are fed heavily and receive proper care!
Have fun working on it and please keep us updated!
Ugo