I am planning to upgrade the code from 9.4.6 to 10.2.4 HF7 on BIG-IP 3600 type C103. HD1.1 is the active boot partition on 9.4.6.

The major difference is that you can only have two partitions, but you can have more than two volumes; also, volumes can be resized if some future software version is bigger than the allocated size.

As to rollback, you can PXE boot or boot from a USB DVD drive and do a fresh format and install of v9. Of course this means losing whatever v10+ stuff you had on the box. Also, the v10 and HF7 installs do BIOS updates, so strictly speaking, your rollback isn't quite to the same exact state as before you started.

I think your config will be copied, but make sure to get a ucs file before you the upgrade.