Move all vermicompost to one side of the bin.  Fill the empty side with fresh damp bedding.  Begin burying food only in the fresh bedding.  As food sources run out on the older side, the worms will move into the new bedding.  Harvest older bedding in a few months, then repeat.

Lay down a sheet of plastic on a table under a strong overhead light.  Dump out the contents of your worm bin on the plastic and divide into several small piles.  Prepare new bedding in bin.  After about 10 minutes, the worms will have moved away from the light down into each pile.  Scrape vermicompost off each pile into a collection bucket until the worms reappear and then move on to the next pile.  Soon the remaining material will mostly consist of piles of worms.  Promptly place in new bedding.  Never expose them to light any longer than necessary.  Five minutes of direct light is enough to kill a worm, due to dehydration.