[Black Tiger]

You can also disable the arcade.
Then upgrade the forums
Then upgrade the arcade to the new forum version
Then re-enable the arcade.

I agree with Marco van Herwaarden about the security, but that risk you always have if you give somebody full access to do something like a board upgrade.
I suggest you change passwords before you give anybody access.
After the work is done, change your passwords again and check the userdatabase for who has admin access.
If possible change the mysql password too (don't forget to change that in the config.php too then), so there is no chance somebody can get in by that way.

I would like to do it for you too, if possible at nighttime (GMT+2) because then I'm mostly active.