Hello Ruchika,
If you haven't done so already, we suggest you go through these materials that will help approach a successful transition:
Unified Interface Playbooks: https://docs.microsoft.com/en-us/powerapps/maker/model-driven-apps/unified-interface-playbook
Unified Interface Transition Checklist: https://docs.microsoft.com/en-us/powerapps/maker/model-driven-apps/checklist-transition-unified-interface
Chapter 3 of the Playbook is specifically focused on the transition process, and offers a lot of tips on how to get the most value from Unified Interface.
As part of this chapter, we also suggest that you embrace the concept of role-based apps.
You can either use first-party apps (such as Sales Hub, Customer Service Hub, Field Service, etc.) or you can create your own model-driven apps.
The benefit is to tailor the user-experience and cherry-pick what your users will need to see (e.g. be able to trim down the list of views for an entity) and associate your apps with security roles, so that different users don't see the same interface and navigation but rather have a personalized experience in the context of their job. All of these apps reside on the same environment, so the data, data model, processes, etc. remain the same!
"Dynamics 365 - Custom" is just a representation of the legacy web client default interface, and you should not be looking at using it when you are on Unified Interface.
Last, the transition to Unified Interface only impacts the rendering of your Dynamics 365 data. There is no risk of losing any data as part of this process.
Henry
