[ioulian]

Sorry to be a buzz kill, but AFAIK you can't "reverse" a worm gear.

[zaarn]

You can, worm gears that can't be reversed are called self-locking, but depending on setup, it's possibly to drive them in reverse with some effort.

[hrydgard]

Can't do it with Lego worm gears though. Maybe if you add some machine oil, but otherwise the plastic-vs-plastic friction is just too much.