@MadhusudanS9253, thanks for your response. To clarify, you'd like to know more about (1) what to expect during the upgrade process to Cosmos from UI-Kit, or are you looking for information about (2) specific types of problems that you can encounter during the transition, and how to solve for them?
We will be planning some content about upgrade best practices in the future.
@MadhusudanS9253 For existing applications built what UI-Kit, adopting Cosmos is development effort, not an upgrade.
Similar to the way Google Material design has component implementations in multiple frameworks including Angular and React, Cosmos has an implementation in our current rules-based Infinity architecture (Theme-Cosmos) and is also being implemented in our upcoming React-based front-end. Which implementation of Cosmos to migrate to depends.
Brand new applications in 8.4 and 8.5 most certainly should be built with Theme Cosmos. The Cosmos design system is our UI/UX strategic direction.
If your existing application is simple (few case types; few rule overrides from UI-Kit; few section rules not based on design templates; minimal skin format overrides and custom skin formats; minimal custom CSS) and App Studio compliant, then you may decide to migrate the app from UI-Kit to Cosmos Theme. See this migration guide. (To understand what you need to do to become App Studio compliant, see the Application > Quality > "App Studio compliance" landing page in Dev Studio)
For an existing application that is complex then I recommend holding off and adopting the upcoming React-based implementation of Cosmos; that way you make a single migration from UI-Kit to the React-based version of Cosmos. To prepare for that, the best strategy is to start with converting traditional section rules to use OOTB design templates (or custom design template if your app has a unique repeatable pattern) and becoming App Studio compliant.
As the React-based implementation matures, we'll offer additional guidance on migration.