Having now migrated to a new Webserver I wanted to update you on what I found. I decided to try it anyhow. I have successfully registered 10.2.1, 10.2.2 and 10.3 ArcGIS Servers with 10.3 web adapters. It was completely painless and has worked a treat. It appears that 10.3 web adapters are backward compatible. Well this is my experience anyhow.
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