When I use /%author%/%postname% in Permalink setting all my old links break

Please help me with this.

I recently tried to use /%author%/%postname% under permalink setting to display author's name in the post link like Buzzfeed but it simply broke all my old post links either shared on social networking sites or through google search engine.

By default , it should redirect all old links to the new structuring but it's not working particularly in the case of "/%author%/%postname%" setting.

Just for the trial I changed the permalink setting to "/%category%/%postname%" , in this redirection was working totally well, even in various other options like day and name, month and name, plain all other settings where successfully redirecting old links to new links.

But as I was inserting author name into permalink setting. It was not working.
I tried Yoast Permalink helper tool ( link : https://yoast.com/research/permalink-helper.php/) But it's valid only when you are changing to "%postname%" setting. It's not valid with author name setting.

Can you suggest me the code to add in .HTACCESS file, when I am changing from "/%postname%/" structure to "/%author%/%postname%/" ?

Please help. Thank you.