You should be able to install this license on another machine without deactivating the previous one, as I believe that you can activate it twice before you can delete the license. To remove ours, I emailed the command in monotouch directly through the website here.
I found that Monotouch email support would be VERY helpful. They are prompt and more than happy to post any problems you have, so I suggest sending them by email if you have any problems.
EDIT - December 9, 2012
Update based on a comment by Kirk Wall : today you can only activate this key once before you must deactivate it on xamarin.com. However, deactivation is just a matter of logging in, choosing your license, and โrecycling your workstationโ. (And you can recycle a workstation no more than 2 times a year).
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