If you can first of all introduce them to grass where there isn't so much of it, and it's not particularly good stuff, it will help a lot. Give them a few days to a week on poor pickings, along with milk and creep, while their systems get used to processing grass but can't overdo it, then, provided they are old enough and are eating at least 1/2lb creep per head per day, you can move them onto good grass and drop the milk.
We have an old chicken run we use for this purpose - the grass is a bit rank, and there isn't really enough for more than a couple of lambs, so we run them in there in batches of 4-7 for a few days and then promote them to the meadows once they have adjusted to including green stuff in their diets.
Just remember, though, that they'll need more creep when the milk stops, or you're forcing them to make up the difference with grass - and that can cause bloat. Most lambs on ewes won't be weaned until 4 months old; we typically wean our pet lambs at 6-8 weeks, so they need something to replace the milk their non-orphaned peers are getting.