The beauty of Avid is that you don’t have to save in and “earlier” version. It’s almost completely backwards compatible. If you have effects or new functions in a new version of Avid that aren’t available on an older version those functions will be missing, you’ll still be able to work.
The best way to take your project from 5.7 to 5.5 is to make a new project in 5.5 under the same name as your 5.7 project, then bring only the bins and media over. It will relink, it will work exactly as 5.7, and when you’re done you can take it back to 5.7 if you want.
We have a MC Adrenaline version 2.6.3 (soon to be upgraded to 2.7), and Xpress Pro 5.7, and an ancient Xpress Meridien 6.1. The Meridien is very different from the MC and newer Xpress Pro, but we take media and bins back and forth without problems. The catch is that the Xpress Pro won’t play all the resolutions of MC, the Xpress Meridien won’t play all the resolutions of Xpress Pro and MC, and it won’t deal with MXF files. But as long as we transcode to the right resolution (or work in the right resolution from the start) and work with OMF media, there are no problems at all. It’s nice.
Good luck! Taking projects from 5.7 to 5.5 will be a cakewalk.
Michael.