The best way to do this is probably to create a new, empty all-in-one project using SDK 2.2.0 and then copy your extensions from the old SDK 3.0 project into the new one, then test.
Why are you downgrading? SDK 3.0 works with versions as old as 4.2, I believe. Hopefully you aren't working with an Alfresco version older than that! If so, I offer my condolences. 
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