Do you have an I-Bus interface? If so use KiwiJochen NavCoder application. If not you can build one. Or check IBUS stuff on http://www.reslers.de

I warn you I can not assure the downgrading-upgrading methos works for Mk-IIIs.

Downgrading is just as easy as upgrading. Follow exactly the same steps and take the same precautions. Just use a firmware disk with a lower version.

It is only done because you want to reload the same versión you already have. The system will not load it becauses it identifies it as the version already loaded. You need thus to load a different version first (downgrade) just to be able to reload the same version you had to start with.

But BEWARE: Downgrading will have some secondary effects you may not be happy with. In USA specs unit you will have the system changed to EURO specs: you will lose your accept screen and you will have feets changed to yards. An no backing from there except through the dealer. You may also lose your telematics info. So you will be needing your I-Bus interface at the end.

And a smart thought: If you can still upgrade, there is no need for you to downgrade.

Hope it helps.