Design a new web service to improve existing solution

I have an existing web service, with 27 methods in all. This is built on a older .net Framework version. I'm looking at upgrading it to the latest version. Authentication and scalability are the key areas of upgrade. Existing web service behavior can be reviewed by interested parties. However, no existing code will be re-used or provided. I'm a developer with 17 years experience myself. So, I can provide guidance.