Good question. I'm wondering that, too. My babies are 3 weeks behind yours and I'd like to switch back as soon as I can safely do it. I hate the clay stuff.
My kittens drag the litter everywhere and also scatter the food a lot. Little slobs!
I was laying in bed last night thinking about this too. I was thinking that if you got a small bag of clay stuff first, let them play and eat it, then after a couple of days when you notice them not eating it anymore, switch to clumping. While letting them play in it, don't be too serious about training them to use it until the clumping is in use. It was just a thought though in my exhausted mind LOL
Usually when the kittens are about 8-10 weeks old I let them use the clumping with no problems. Watch them for a week or so and see if they still are "tasting" the litter. If not, then it should be ok in a few more weeks.
Mine are 3 to 4 weeks old. I just bought clumping litter yesterday, not realizing that they need non-clumping. I bought the non-clumping ones today and took my time setting it up. I showed Batman (my brother named him btw ) his new litter, and of course, he walked over to the clumping litter and did his deed. My brother then tells me after I've set up the new litter that the little kitties have been using the clumping ones with no problems!
They will use the clumping and non clumping with no problems but if they eat the clumping they can get blockages and die from it. Make sure you use the non clumping until you are sure they aren't eating it anymore!