If a developer had previous customizations on 2019.1 core files can i easily bring them over to the newer version of SCA ? Whats the process like these days? i have not ever used the core files since Kilimanjaro

Depends on the customisations 🙂 The two big things since 2019.1 is that we no longer recommended using Backbone modules directory (use the extensibility layer ones) and we now use TypeScript