I'm not sure the best way for a full compost pile.

Here is how I have been harvesting my worm bin (it is a bit tedious if you try to save every baby worm and cocoon but works well for the adult worms.)

I have a 2' x 4' bin. I started my 2# of worms in only one half of that bin. As the population grew and the first half started to build up some castings I added bedding and food to the other half of the bin and quit feeding on the original half hoping that many worms would move over to the new half and make harvest easier. When I finally decided it was time to harvest I did the following.

Scoop a pile of castings/compost onto a tarp or other surface.
Leave it in bright light for a little while so the worms will burrow deeper.
Scoop the top portion of the castings off the pile for use. As I said, this works well for the large worms that seem faster to move away from the light. It is still likely that the cocoons and babies will be in that upper portion of castings and if you take the time to pick them out, it is tedious. Then again, in the right climate they will survive in the garden provided there is mulch down to feed/protect them and enough moisture.
So, eventually with this scooping off of the top of the piles you will end up with a smaller pile with some castings but mostly worms, this can be put back in the worm bin (compost pile or worm manure stall) with fresh bedding and worm food.

I hope this is what you mean by migrate them since I'm not sure how you could get them to migrate on their own unless the compost pile happens to be in the stall next to the manure stall.

You might also consider keeping a worm bed in the barn year round and then you don't need to worry about rescuing the worms from the compost pile to move to the barn for the winter.

here is something from a website I found that might work for you. I'll put the link below, but here is a sample of the wedge system that would help you without having to seperate the worms. Having some cattle you could do this easily. Make the size dependant upon amount of cattle you have vs. the worms you have.

WEDGE SYSTEM

This modified windrow system maximizes space and simplifies harvesting because there is no need to separate worms from vermicompost. Organic materials are applied in layers against a finished windrow at a 45-degree angle. The piles can be inside a structure or outdoors if they are covered with a tarp or compost cover to prevent leaching of nutrients. A front-end loader is used to establish a windrow four to ten feet wide by whatever length is appropriate. The windrow is started by spreading a 12 to 18-inch layer of organic materials the length of one end of available space.

Up to one pound of redworms is added per square foot of windrow surface area. Subsequent layers of two to three inches of organics are added weekly, although three to six inch layers can be added in colder weather. After the windrow reaches two to three feet deep, it can be extended sideways by adding the next layers at an angle against the first windrow. Worms in the first windrow will eventually migrate toward the fresh feed. Fresh manure is added to the second pile until it reaches the depth of the first one, and then a new windrow is started. Worms will continue to move laterally through the windrows. After two to six months, the first windrow and each subsequent pile can be harvested.

The wedge system is one of the processes used on the Yelm Earthworm and Castings Farm, located near Yelm, Washington. The 10-year old worm farm is among the largest in North America, with 30,000 square feet of enclosed space in addition to production areas outdoors. About 15 tons of redworms are used to process a variety of feedstocks, including animal manures, wood chips, leaves, spoiled hay/straw, grass clippings, yard debris, produce, food scraps, waxed cardboard, soiled paper, and corrugated cardboard. Jim Jensen, operations and development manager since 1997, describes how the wedge system can be used for vermicomposting on dairy or horse farms in his KISS (Keep It Simple & Save) plan, which can be found on YelmÂs website at www.yelmworms.com

Here is a link that might be useful: ways of vermicomposting

When spring or summer gets here and you get ready to pull out some worms just quit feeding all over the pile and feed new stuff in one corner only and all of the worms will eventually congragate there where the new food and water is. Just give them a few days and scoop them out with a pitchfork. Put in some more fresh feed and the next day they will pile up there again.

wfike, that is kinda what I thought with my bin, that I would quit feeding in the one end to get them all to move on so I could harvest the castings without alot of worms in em. Didn't works all that well, It had been months since I had fed anything in that end of the bin and yet there were lots of worms still in the almost pure castings even with plenty of bedding and food in the other end of the bin. Aparently there is still plenty of bacteria for the worms to eat again and again even when there is very little food or bedding left visible to our eyes.

You could put a cover over the area that you want them to go to. They love to get on top of the bed under a cover to feed. Does the whole bed have moisture in it. You might make a box like bin at one corner down in the bed to keep any moisture that you add in that area only. The worms will leave the dried up area for the boxed area with moisture and food. Put a mash bag under the cover with something that they like in it like cantalope or pumpkin in it. When they fill it up just remove it and put it on the other bed in the daylight and they will go down into the new bed. Then move the trap back to the old bed for the night and do that every day.