CA Access Gateway Parallel Upgrade

Document ID : KB000126334
Last Modified Date : 06/02/2019
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Is there any document available for parallel upgrade of CA Access Gateway?  We don't want to disturb existing environment (12.52 sp1) and want to migrate to 12.8. 

We already read this document:

...but it only talks about in-place upgrade  How is a parallel upgrade performed?
There is no parallel upgrade available for Access Gateway (formerly Secure Proxy Server).  To achieve the same effect, you install a new instance of Access Gateway and then manually transfer the needed settings from the old installation to the new.