Did you ever feel that it is going to be very difficult to upgrade Angular5 project to Angular6.x version. It might be "Yes" for many developers. Even I faced many errors while upgrading my PrimeNG-Extensions project (https://github.com/sudheerj/primeng-extensions) to Angular6.x version.
This is because the schema will be different for .angular-cli.json and angular.json. Instead you need to follow the below four steps to achieve successful upgrade.
Run the below npm command to convert the project configuration file from .angular-cli.json to angular.json file. It will also update unit testing and tslint config files such as karma.config.js, tsconfig.json and tslint.js etc.
You also need to upgrade older TypeScript version which may not be compatible with latest Angular version. So manually update this dependency from "2.4.2" to latest version( currently it is "2.9.2"). Another important upgrade is RxJS version from "5.5.2" to latest( currently it is "6.3.2"). Other third party dependencies upgrade which depends on your project also should be done based on the usage.