Spare Parts by Sally Rogers Davidson.
C-grade citizen of the Greater Melbourne Megalopolis, Kelty lives in a city filled with towers reaching halfway to the sky. While 'Skywalkers', the A- and B-grade citizens live above the clouds with access to all the wonders of the late 21st century, 'Subbies' like Kelty must dwell in the shadows and smog of the streets below.
When her best friend is horribly injured in an explosion at the recycle plant where they both work, Kelty is faced with the loss of a friend and a hopeless future, or the unthinkable choice of leaving everyone and everything behind to join the Space Corps. There's just one catch - first she must trade her human body in for a State-of-the-Art Cyboform.

First published by Penguin Books Australia in 1999, Spare Parts was short listed for the 1999 Aurealis Award for Best Science Fiction Novel, and chosen as a Notable Australian Children's Book for the Year 2000 by the Children's Book Council of Australia.