I am trying to upgrade an old laptop with the latest OS from apple. Sadly, the current upgrade program is unable to do so if the main partition/volume/container is encrypted.
Add a new Volume with Disk Utility
In disk utility click on you current container then click the little plus to create a new volume:
Now type a new name, eg. "monterey", select
APFS (Upper-/Lowercase) and confirm with
Continue the installation
Back in the Upgradeer the new volume should have been recognise already. Select it to continue the process while keeping your data secure.
The biggest question is, can the new volume be encrypted in the future to secure the OS as well as the data?
At the time of writing, I can not answer this. I will look into it in the coming days. Consider subscribing to my RSS/Atom Feed so you don't miss it.