Thanks Dennis,
For the benefit of anyone else using C# and an API key to access the API from an external application this is all you will need to change.
I created a private app - assigned scope and the creation generated a token.
This is an excerpt from the original code (using RestSharp)
hapiKey = “your_api_key”; // real app reads this from a config
var client = new RestClient(baseURL);
var request = new RestRequest(“crm/v3/objects/companies/”, Method.Get);
request.AddQueryParameter(“hapikey”, hapiKey);
// etc..
This is what you need to change.
token = “your_token”; // real app reads this from a config
var client = new RestClient(baseURL);
var request = new RestRequest(“crm/v3/objects/companies”, Method.Get);
request.AddHeader(“Authorization”, "Bearer " + token);
// etc.