How do I 'downgrade' an Adobe Premiere Pro project file to open in older versions?
Since this is the top google result, I would like to give the actual solution to this problem



Option 1


Duplicate the .prproj file you want to downgrade and name it temp_downgrade.prproj or something. Just so you don't have to mess with the original.
Download 7zip.
Right click on temp_downgrade.prproj and select Extract here.
You should see temp_downgrade next to the original temp file with no .prproj.
Now right click on the temp_downgrade file and open it with a text editor. 
You should see a couple million lines of confusing text, ignore it. you are looking for the 4th line, it will look like this:


&lt;Project ObjectID="1" ClassID="62ad66dd-0dcd-42da-a660-6d8fbde94876" Version="29"&gt;

That version=29 (or whatever it says in your file, could be any number) is what you have to change. Simply set it to 1 so the whole line reads:


&lt;Project ObjectID="1" ClassID="62ad66dd-0dcd-42da-a660-6d8fbde94876" Version="1"&gt;

Save the changes you've made to the file.
Now right click the temp_downgradefile - remember the one without the .prproj - extension and select add to archive under 7zip .
Change archive format to gzip and change the file name to be something. like temp_downgraded.prproj. Doesn't really matter it must end in .prproj though.
That's it open temp_downgraded.prproj like a normal premiere project and it'll ask to upgrade the file or something. Save the copy somewhere permanent as that's where any changes you make will be saved. You can delete all the temp files now. 


Here is a gif of the whole thing. 



Option 2

EDIT 2018: This tool mentioned in an answer below mine seems great! http://joshcluderay.com/downgrade-premiere-project-converter/

If you would prefer a local open source version, I also made a small JavaFx app that should automate everything in Option 1. You can download it for a Mac/Windows computer with Java(!) installed. Over here. 



Are there any consequences from doing this?

Not reeeeeally. At least - it's much less than the current accepted answer on this thread. If you export something as an xml file on premiere you will effectively lose most of the editing you've done. Aside from maybe the most basic sequences. All the colour correction, Sound work, etc is gone. With this method - yes some of that will disappear if you are working on an old enough version of premiere. But, I'm talking like five years old. If the difference is CC2017 and CC2016. Sure you'll lose any effects that only exist in CC2017. But I would bet this is mostly niche stuff and 99.999% of your project will probably be fine! But - even if it isn't - say you make some small changes to your downgraded version on CC2016. If you copy the downgraded version to a computer with CC2017, after the changes have been made. All the newer features will still be there. So as long as you render on the computer with CC2017 nothing will be lost.

tl;dr: Obviously there are some cases where you could lose information, but if you just need to make minor edits or fix something small on your home computer then take it back to work or something - this is fine. 



Why this works?

Apparently adobe save files are just gzipped xml, so that's why. Should work on AfterEffects as well.