Adjustable converter

snakebunk

I don't know what to call it, but today I came to think about how nice it would be with a converter that could switch between 1.4x and 1x, or another combination. Kind of like in the Canon 200-400, but as a separate converter. Is there or has there been anything like this?

Pistnbroke

You will find some 1.4 to 2 x on ebay but these are old