Questions about Chilean Myrtle bonsai

A Minnesota bonsai friend sent me an email today asking me to post these questions on the forum:

" The tree is a Chilean Myrtle (Luma apiculata) about two feet tall with small dark green leaves. It's growing like crazy right now, and looks happy in the full sun. I'm wondering about repotting : it's in plain old potting soil ..... how much root can be removed the first time it's moved to bonsai soil? Is this a tropical? General bonsai care and training tips for this species would be appreciated."

My friend sent these websites:

http://en.wikipedia.org/wiki/Luma_apiculata

http://www.arthurleej.com/p-o-m-Dec06.html

Many thanks for any help you can give!
Norma