Upgrade from 8.12.5 to 9.6.6

Hello, If we have a project with two branches one is 8.12.5 version and another is 9.6.6 version ! There will be a lot of UI related changes (Added different Atlas, different Layouts to pages, CSS etc) in 9.6.6 (Created with old revision of 8.12.5) and a lot of implementation in 8.12.5 (Has the latest revision). What are all the ways we have to merge all the changes from 8.12.5 to 9.6.6 without braking the UI changes in 9.6.6?          
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