How the two downgrading lines differ
Premiere and After Effects are two independent processing lines here, not one tool with a different extension. The differences show up in three places: where the work happens, how far back you can go, and what you get back.
Where the work happens
The Premiere line is handled server-side after you upload and returns the result in one go. The After Effects line runs entirely in your browser, so the project never leaves your device. Both process once and keep no long-term storage.
Different reach
Premiere covers more ground with 14 target versions, the earliest being CS6. After Effects currently offers 9, the earliest being AE 2018. That gap reflects each line’s capability boundary, not an oversight.
What you get back
Both hand you a new project file that opens directly in the older software. Both remove whatever the target version does not recognise — the principle is the same on either line.
Which one to use
The file extension decides it: .prproj goes through the Premiere line, .aep through the After Effects line. They are not interchangeable and do not need to be.