[cypher]

I would keep back some of the shavings from the cage before the switch (don't clean too soon before so it has his scent on it) use mostly fitch then put a layer of the old shavings on top then once he's adjusted you can gradually clean away the top layer if it doesn't all get mixed in with burrowing until you're just using fitch.
They generally love fitch so as long as you don't remove his scent completely (don't clean toys, wheel etc at the same time as changing) he should be fine.