I'm stripping down the ex-tropical tank and there is a good population of red worms in the coarse sand, what is the best way to harvest them so I can move them into the 430L that is the new tropical (without putting all the sand in there)?

Probably turn off flow and any oxygenation of water and they may come out onto top of sand to be vacumned up

Put the sand in to a bucket with a lid to exclude the light. Keep tipping off the water and when they run out of O2 the worms will come to the top and you scrape them off. Put them in to another tank with about 10 mm of very fine gravel and feed them on kitten pellets. Soak the pallets and break them up so they spread evenly. Add a sponge filter and aerate. Siphon of some gravel and worms once established and the worms will ball up in the bucket separately. You now have a permanent supply of Lumbriculus for you fish.