Hi Luke,
Change bars are definitely supported by Arbortext. The thing is, they are not enabled in any of the out-of-the-box document types, including DITA. There is an open idea to implement such support for DITA: Add revision bar functionality to .ditaval files
The "step by step" to enable change bars is quite complicated. You would need to extend the basic DITA stylesheets (using Arbortext Styler) to add support for the @rev attribute. You would then find that Styler has no user interface to the change bars feature, so you would have to add "Edited Source" for handling the @rev attribute. This Edited Source would require you to write Javascript code (for APP engine) in order to enable change bar display.
Here at GPSL (www.gpsl.co) we are a PTC partner who specialize in this type of work. We can configure, customize and integrate Arbortext software to meet the most demanding business requirements. We are also able to provide training for this type of work.
Cheers,
Gareth