sedirea japonicum needs to be repotted

[Talila6]

hello: i acquired this siderea japonicum in california this spring. it bloomed beautifully (was in bud when purchased)
now summer is upon us.. finally!
it looks like this plant needs to be repotted. I would very much like to rise to the challenge and have it rebloom for me next year.
it is now planted in a deep container, the medium looks like some basalt stones and maybe bark.
the roots were out of the pot as shown when i purchased this plant. the plant has grown more roots since.
the roots are kept very moist by keeping a little water on the bottom of the tray where I have placed some pebbles. it seems to like this arrangement.
does anyone have a suggestion for best chance of success in repoting this plant?
i have had suggestions to mount it.. but i am not sure that i will be able to keep the moisture level that this plant needs.