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Back to the original question...
Addins are ArcGIS-version specific. A 10.2 compiled Addin won't work with 10.1, even if the underlying ArcObjects code is 10.1 compatible. It's also true vice-versa.
If you have clients running 10.1 you may need to keep an installation with ArcGIS 10.1 & Visual Studio 2008 or 2010 in order to develop new solutions for them.
But, if you have the ESRI license, you can always use a virtual machine software to spin up another instance of ArcMap on the same machine, assuming your machine is capable with resources.
EDN server Software and Data may be installed on multiple computers for use by any Licensee
EDN developer; all other EDN Software is licensed as a Single Use License.
"Single Use License" means a license that allows Licensee to permit a single authorized end user to install and use the Product on a single computer for use by that end user on the computer on which the Product is installed. Licensee may permit the single authorized end user to install a second copy for end user's exclusive use on a second computer as long as only one (1) copy of Product is in use at any time. No other end user may use Product under the same license at the same time for any other purpose.
Leo, why switch Java? Someone should write a technical article Top 10 reasons to Program ArcObjects using Java