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You can probably get away with it on many apps. It's as simple as renaming any other file...slowly double click on the app's name and just type something new.

But I can think of some apps where upgrades look for the previously installed version and won't be able to find it if it's been renamed. A notable example was the Safari 3 beta...if you had renamed Safari 2, the 3 beta installer wouldn't be able to find it and wouldn't allow you to install the beta.
 
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