Huh? Just create a new slot, set it in the boot section to boot, install your new ROM there.
Why "move" it? SLOT 2 can be your primary slot once you set it in the boot menu.
No need to back up slot 1, create slot 2, restore to slot 2, switch back to slot 1, erase and create partition in slot 1, install new ROM in slot 1.
You're making it harder than it really needs to be!