I have been running Scrivener on my iMac (OS X 10.11.6) and my MacBook Air (current version of OS X). I’ve downloaded Scrivener 3 to my MacBook Air and went through the file conversion. Scrivener 3 does not support the version of OS X I’m running on my iMac (early 2008 edition but I hate to give it up). Will I damage the file structure if I attempt to open the current project file (after conversion) using Scrivener 2? Thank you in advance for any responses.
Scrivener 3 can export a Scrivener 2 project. That’s what you’d need to do to use the older machine.
The two biggest changes in Scrivener 3 are the Compile function and Styles, neither of which Scrivener 2 supports.
Katherine
Thank you for the quick response. Now I have a good reason to get a new iMac.
That said, and not to ruin your excuse for a new iMac 
, although styles will be lost in round-tripping with Scrivener 2, the compile settings you create in 3.0 will all be maintained and available again when you re-upgrade the project. So if you need to work this way, just avoid styles for the time being and do all your compiling on the newer machine.
This is great news. I did receive a message that the file I was attempting to open was created with a new version of the software. My guess is that I will either need to go to a version 2 backup or export from version 3 into version 2 format. I will attempt that later this evening. From your reply I gather you may also be going through this process and I noticed you use the Windows version as well. I too have the Windows version, but have not yet attempted to open the version 3 files from that platform. Do you have any experience in that process as well?
Thank you in advance for your response an please accept my apologies if my questions seem basic, but I have limited experience with Scrivener (but love the app).
I’m sorry I wasn’t clear.
Scrivener 2 for Mac cannot open a Scrivener 3 project. Neither can Scrivener 1.9.7 for Windows. The Scrivener 3 “Export Scrivener 2 project” command is the only way to make your work accessible to the older versions of the program,
Katherine
Following up again from Katherine, since I think I’m the one who added to the confusion: You will need to use the File > Export > as Scrivener 2 Project… feature in 3.0 in order to work cross-platform with Windows 1.9.7. The export function will make a copy of the project in the older format that 1.9.7 can read, and after you have worked in 1.9.7 you will then need to upgrade the project again to 3.0 to work on it there. So the process will be continually making new copies and you’ll always be working from the latest in this trail, but it does work if you need to be using both 3.0 and 1.9.7 at this time. Just be sure to keep your copies straight! 
Since 3.0 includes a lot of features that 1.9.7 does not have, not everything available to you in 3.0 will be able to survive being downgraded and edited in an older version. Styles is one of these–this is written directly into the RTF format that is the basis of text documents in Scrivener, and as such it’s not supportable in the older version at all, so you’ll lose styles in your text trying to work back and forth like this. But the compile settings in 3,0 will be saved; they’re ignored entirely in the older version, but will be present again when you re-upgrade the project. So if you stick to Mac 3.0 for compiling, you’ll be fine with this and not lose anything in the process of exporting and re-upgrading. Custom metadata is also saved as much as possible–the new list, date, and checkbox types don’t exist in 1.9.7 and will be converted to text metadata there, but as long as you stick with the same format (e.g. use only “yes” or “no” in a metadata field that should be a checkbox), they’ll be converted back to their original types when upgrading back to 3.0.
(I’ll refer you also to section 3.4.3 in the Scrivener 3 user manual, available from the Help menu.)
I hope that makes sense! In sum–it is possible to work this way, but only by exporting a Scrivener 2 Project from 3.0, then re-upgrading to 3.0 after editing on Windows.
Thank you to all who replied. I am sad to say I did not get a new iMac 
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I am happy to say I am running the current version of OSX on my 2008 iMac and of course, running the new version of Scrivener 
Thanks dosdude.