Tell me about transitioning from nursery stock soil to bonsai mix in Juniperus procumbens and horizontalis…

[raenstorming]

You only mess with 1/3 of the roots at a time. I got that. But how do you maintain even moisture when the new medium is going to dry out faster?

My first attempt at this was from a 1 gallon container to a 10" pond basket, and in hindsight that was pretty dumb. The outer mix surely dried out way faster than the inner older mix, and it didn’t needed fast draining bonsai mix in a pond basket while still in training. Is it less of an issue if you, say, stick with the original 1 gallon container and only change the bottom third of the soil?

 

[Wires_Guy_wires]

If the plant is not really old, I'd go with changing 50% of the soil, or all of it in one go. In the 50% case, scrape the remaining 50 around the edges and backfill that area with bonsai soil.

Top and bottom approaches can make sense, but I feel like once you reach the second repot, there's no way to stop it from falling apart and budging you back to a complete bare root, more or less.

Procumbens and horizontalis are tough. I've never had any issues barerooting those. But none of my specimens exceeded 10 years in age.

 

[River's Edge]

You only mess with 1/3 of the roots at a time. I got that

If you post a picture the advice can be more tailored to your specific situation. Where the advice changes is on the age and condition of the plant. It also changes depending on what you mean by mess with the roots.
To simplify, if your primary goal is to change out the soil and the plant is young with good roots in healthy condition. This situation would allow one to sort out the roots, replace most of the nursery soil and replant. Note I did not mention cutting the healthy roots at his stage, just changing out the soil. While in this process remove all dead roots, rocks and organic material from the root ball. This approach ensures equal drainage throughout the root ball and an opportunity to improve the root positioning. By limiting the root pruning you are recognizing that juniper prefer less root work. If needed longer roots can be folded under. Often referred to as the " lasagna method"