How to install over an older system but keep disk partitions?

Hello, I have an old FreeBSD system (running 9.3-RELEASE) that I would like to update to the latest 12.0. Trying a "live USB" it seems that the new system would work OK. In the current installation I have a specific partitioning scheme on the boot disk (SSD) and several HDDs configured for ZFS. I would like to keep this partitioning scheme and of course still be able to use the data in ZFS.

What is the process to make a clean install (rather than step-by-step upgrade) of a new system while preserving the old disks?
- Will it work to backup and then restore in the new (clean) system? Which files? This especially applies for not loosing the ZFS!
- Alternatively will it work to go ahead with the installation process on the boot disk but I don't touch the HDDs (ZFS)? how do I regain access to it (how do I make it part of the system again)?

Many thanks!
 
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