Difference between .net core and mvc5

I have a big application Cloud based SAAS my first developer uses Agnular 1.6.8 and service oriented architecture i have given him a template as well for UI, Now I end with the previous developer and the new developer is using Angular 5 with .net core , as i have spend much amount on previous work and it is approved as well just the technology was not used as requested , so can somebody tell me how much my previous code can be reused in new .net core application , all bsuiness logic is in angular controller so i think he can use it at maximum lavel but he is saying it is not usable can someone share the information.