Vivek,
You can import a CRM 2011 solution (managed/unmanaged) in CRM 2013 environment. It will auto upgrade the solution to CRM 2013 design. Once the import is done you will be able to switch to CRM 2013 forms. However, you cannot go back to using the upgraded
solution files in CRM 2011 environment.
You will need to test your solution thoroughly to ensure the functionality isn't breaking. Scripts/plugins/custom WF / pages etc may be using components that are no longer supported in CRM 2013.
HTH
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