Might be worth clarifying with the vet as it takes at least 24 hours for worms to be shifted by wormer so if you move straight away you will take them with you and the idea is to leave them behind!
I think the vets reasoning is that if you worm and hold before moving to fresh pasture, what the animal takes with them will be those worms resistant to the wormer. You then have sheep grazing land with only resistant worms on it ......
by worming and moving straight away, you reduce the overall burden, without selecting for resisant worms. If you get my drift.
We are worming in the shed at the moment after they've lambed, but before they go out onto grass, as you are doing. Just interested in what others are doing.
Mike