Upgrading standalone 448D, and not losing the entire config.

I'm looking to upgrade a few FortiSwitch 448D that are running on 3.6.4. I've uploaded 6.2.5 to the secondary image slot and clicked upgrade. Switch boots up with defaults...[&o]

I've dug around and come up empty on the procedure to upgrade a switch and retain it's config. Any advice?

Oh and reverting back to primary image boots 3.6.4 w/ config in place at least.