I believe that you are working on translating the opening balance movement of a balance sheet account. Is this correct? Can you give more details on how you are doing that in you application?
Based on my experience, the best approach is to make have a restatement of 2012 and post the new journal in 2012. The new opening balance will be copied to 2013.
If this is not possible you can create a new member OpenBalAdj in order to post the new value of goodwill. This movement should be translated with opening rate and not the closing rate. In the FXAdj member that you created, you will post the translation difference between opening rate and closing rate for the value that you post in the OpenBalAdj.