I would be surprised if there is an official document out there to guide you on this. Most of our documents are to support you in an upgrade (Not hold you back by keeping old, deprecated, or legacy features/content after upgrading your platform.

In other words, I do not think such a document exists and we heavily promote and recommend you perform a "true upgrade" so to speak and embrace the new portals, features, look and feel, etc.

It will net you better results in the long term to make the initial investment in a proper upgrade, rather than to spent time, effort, SRs, etc getting old things working again on a new platform.