Forget which client certificate is used by Chrome for an URL
Restarting the browser will help and ask you again.Looking forward to see better solutions.
Open the target URL in an incognito session, and it should re-prompt. Not a great solution, but to me it beats having to restart all of Chrome, especially if you already had a lot of important tabs open.
This particular thing has sucked since the beginning of web browsers. The only solution is to use the platform APIs to build extension to manage client certificates. This is in fact what Google recommends but it's mainly for CAs and other solution providers. So the only way is to reload the browser or use incognito windows.