What downgrading actually does
In plain terms: everything the older version cannot read is cleared out, so the project opens properly in the target version. You do not need to know anything about the file format.
Identify the source version
The project is read to determine which Adobe release it came from — that decides which versions it can go down to.
Work to the target version
Anything that does not exist in the target version is removed, along with the references it leaves pointing nowhere.
Produce a new file
The result is a new project file. Your original stays exactly as it was, from start to finish.
Open it before you deliver
Try the result in the target version before delivery. That step is more reliable than anything written on this page.
Standard, or a little steadier
Standard processing
Converts as completely as the target version allows. For when you want to keep as much as possible and will verify the result yourself.
Steady mode
Prioritises a result that opens in the older version, converting less if that is what it takes. If you are unsure, start here.
We do not promise that every project can be brought down
Downgrading is inherently lossy: whatever does not exist in the older version can only be removed. Whether it succeeds depends on how much of the project relies on features the target version does not have — no tool can change that.
Rather than overstate things, we put the expectation up front. You can verify it yourself: open the result in the target version, and if it does not work, try a target closer to the source version.
And please keep the original project
Downgrading is one-way. What gets removed does not come back when you upgrade again. Before you start, copy the original project somewhere else — this advice matters more than everything else on this page.