Issue: you have completed the installation of a new SSL cert on IIS however the server is still serving the old certificate rather than the new one.
Additional factors: IIS appears to ‘cache’ the old existing certificate, even if you delete the old cert from IIS Manager or you delete the old cert from the personal certificate store on the server.
Solution: I’ve found a solution which does not require re-starting IIS or other downtime, which is to manually specify the certificate that should be used by the web server. By default it automatically chooses what it thinks is the best certificate, by manually specifying you can force it to use the one you prefer.
Next, follow these steps to manually specify new SSL certificate in IIS.