Updating applications purchased by another user? How?

[SlapMonkey]

When the 2.2 update was released, one of the undocumented features was the following:

When updating an application on an iPhone or iPod touch, if the application was purchased by another user, it shows a dialog to change the user and input the password. Prior to 2.2, the App Store didn't even show the update of a application if the account configured on the iPhone or iPod touch was different.

Unfortunately for me, I am unable to get this to work. Out of the 200+ apps I have, I only have one that was somehow accidentally purchased with another email address/Mac account of mine and it just so happens to be my most expensive app, Beatmaker, that needs updating. Also, my Mac just completely died on me so I have no way of updating this through iTunes on a computer, so I MUST get this to work on the iPhone...

So when I click on the new Beatmaker update, instead of giving me an option to login with another user account, it simply says:

This discounted price is only available to customers who own a previous version of this item. To purchase this item at the full price, click Buy.

... and when I click the "Buy" button (thinking it will still give me the option to possibly login to my other account) it gives the error:

This item is currently being modified. Please try again later.

So has anyone had success updating an app on your iPhone that was purchased with a secondary user account and if so, what am I missing? Any help would be greatly appreciated!