I just started growing rhubarb this year, but the soil under my raised bed turns out to be clay and sand with glass and stones. The roots did not go deep, but tried to grow out under the sides of the bed. Consequently, they were unable to hold their own weight. I twisted all the stalks off at the base and proceeded to add almost 2 1/2 feet to the depth of the box and paint it. While I was doing that, all the plants have been growing like crazy, and now at almost the end of September, I have six inches of growth. Now what? Add the new soil on top and replant on top? Wait until the frost and recut the plants, then cover and wait? Dig it all up and dispose and start over?
I would wait until the plants are dormant and transplant.
You can do this in the fall or wait until early spring.
Here are some links with more information.