No, it's not possible to change renderer at runtime. I would be interested to see any examples of games running slower in WebGL mode if you could share them. I've run several pretty different tests and it can often double the framerate or more on some systems and is never significantly slower.
If you're *only* seeing slow rendering in IE11, it's probably using a software renderer, which is likely unique to your system due to buggy graphics card drivers. Try updating them.