Changing mu to h works totally fine for me. You can even change from mu to x but I think x is for mobile only. Just to clear any confussion, I hope the classic interface you are talking about is same as the basic html view. You can switch between standard view and basic html view from the buttom of the page. There are links there. But here's a link from Google so try this: https://mail.google.com/mail/u/0/h/1pq68r75kzvdr/?v%3Dlui

Make sure you are logged in before clicking on the link above. Let me know if I can still help.