To switch from AOSP9-based Sailfish port, you would have to reflash the device. The following procedure is suggested for the devices switching over to this version.
Backup all your data. I would suggest to backup your home directory
fully with whatever tools you feel comfortable with. I have used
rsync for it.
To make it simple to switch back if something will break badly, it is possible to make filesystem level backups. For that, you would need PC, SD card which is large enough to keep all your storage, and recovery image to boot into recovery mode. See documentation on how to do that.
To ensure that you have the latest updates for firmware, such as modem and bootloader, get the latest version of Sony Stock Android. You could use XperiFirm to download it and Newflasher to flash it.
Notice that Newflasher is using flash mode (not fastboot), so follow the instructions on how to flash it.
After flashing stock, boot into it. You don't need to fully set it up, but you would need to establish connection via SIM card and let it update the modem if needed. So, just keep it running in Android for a bit.
Follow flashing guide. Notice that you will need to download new OEM images for AOSP10 based port.
Note that this has not been tested, but is expected to work.
If you have filesystem-based backup, getting back to AOSP9-based port should be simple. Flash OEM Tama image for AOSP9 and flash AOSP9-based port latest Sailfish release. Boot into Sailfish as it will adjust LVM filesystems. You don't need to setup anything, just skip all the setup questions and get into the desktop. Shutdown the device after that and get into recovery. See documentation on how to restore your backup.