There is no permanent way to switch, but you can easily use a URL to go to the original classic as the servers are still up for it .As a level 1 I cannot post URLS, can only comment them.
I've tried it on numerous browsers, numerous accounts, numerous computers, but the LYING TROLLS and BOTS refuse to tell people the truth. They will spam LIES rather than tell people the truth.
There are three versions of yahoo mail, all three can be accessed via a direct URL. (New mail will throw you back to basic if your browser doesn't support it) [Oh I see, you can access basic and classic at anytime, but only access new mail if and only if you've set your account to "Switch to the newest Yahoo Mail")