Q:  I started a project in firmware 17.2 for a compact. Can I change back to 16 if we want to?

A: Save your v17 file as .L5K format.  Open the file in a text editor and change the "major" v17 to v16.  Then open the .L5K file with RSLogix 5000.  You may encounter an error or two as you have to make a few changes but this does work.  Starting in v17 and looking forward (e.g. v18 down to v17) will be supported as an option in the controller properties.