A little help with my japanese quince and a prunus incisa oshidori?

[Carapace]

Hi, I am getting a a very japanese quince and also a japanese cherry blossom "oshidori" and I have read somewhere that these plants should be repotted in the autumn.
I am not really chasing flowers next year, I want them to grow well and get established and also avoid diseases that usually affect these plants, which are more common in the early spring time from what I know?
Also, if the answer is "yes, you should repot them in the autumn", 1.should I remove all the nursery soil at once, 2.how much root can I cut off just in case they are very very bad (don't think they will be).

 

[Bonsai Nut]

It depends is the best answer in these cases.

I would generally NOT repot a quince or Japanese cherry in the fall. I would wait for the spring.

And to your other questions... if the tree is already established in bonsai soil, you can be more aggressive. But be really careful if the tree is in nursery soil (perhaps rootbound) and you are moving to bonsai soil. If this is the case, be cautious, and it is more important to trade out soil slowly, and cut roots less aggressively.

 

[Carapace]

Good news, I repotted the japanese quince 4 days ago, took off all the nursery soil without needing to cut any root and so it still has 95% of it's roots, potted it up with a nice free draining mix (not the final bonsai soil).
It is looking good, no drooping leaves or anything, I would say it was a succes.