Might be a truism to the experts, but just in case anyone’s interested: I just succeeded in switching from Fairphone OS Android 10 on the FP3 (down) to /e/OS Android 9 using the same instructions I used when I previously switched from Fairphone OS Android 9 to /e/OS Android 9. No prior downgrading of Fairphone OS Android 10 to Fairphone OS Android 9 necessary.
Bear in mind what I just described is a downgrade (lower Android version), so data loss is not a risk, it is guaranteed. So if you want to keep personal data from Fairphone OS Android 10 and downgrade to /e/OS Android 9, you must make a backup.