I don't think full metadata roundtrip is possible - it depends on what was done in Pro Tools as well as number of additional tracks, etc. Complete fluid, flexible, and automated Change Management would be a fantastic feature but a complete solution is a challenging task. Containers ala Avid DS might help the process in the interim, but there is no support for that.
Typically, the Media Composer would make changes on scenes or mix down from Pro Tools and a change list is created for picture that is handed off to the sound editor to make the changes.
When exporting an AAF from Pro Tools, enable the Enforce Avid Compatibility option to limit the sample rate options to 44.1 or 48 kHz and enable exporting of clip gain data.
When you do this, you have to realize that you may end up with hundreds of subclips named Sample Accurate Edit. These subclips represent the difference between where the Pro Tools clip was edited and the nearest whole frame. If you do not want these Sample Accurate Edit subclips in your timeline, then you need to force the Pro Tools editor to cut everything on the time code frame edge. This is not a preference, so you will have to tell the editor to be cognizant of this when editing.
If you can deal with the Sample Accurate Edit subclips then you can do some round-tripping with MC. Plug-ins and their automation will not make it back into MC.
Typical workflows involve the Pro Tools editor doing a manual conform or using programs such as Conformalizer or Virtual Katy to take an AAF, ALE, or Change Note and use it to conform the Pro Tools session.
Jonathan S. Abrams, CEA, CEV, CBNT
Apple Certified - Technical Coordinator (v10.5) Support Professional (v10.6 and v10.7)
Vice-Chairman, NY section, AES