Switching litter

Hi everyone,
I have a question about switching litter. I took Sparta to the vet on Monday because her eye has been watering a lot lately and she's been sneezing and the verdict was URI. I know that's caused by dust from clay litter, which is what I was using, so I decided to try a different kind and Tuesday night I got them Swheat cat litter. It's wheat and supposedly dust free and when I put it out for them, Sparta used it but Ares sniffed and played with it but wouldn't go. I started to worry but then he actually went so I figured no problem, they like it. But this morning Ares was meowing like mad and sniffing at the litterbox but not doing anything. Then I found pee in the cat bed. So we let him have access to the clay litter and in he went for a poo. So I'm wondering if there is a good way to switch litters? Should I leave both litters accessible and let them choose? I'm afraid of doing that because what if they only keep using the clay one? I'm not sure what to do. I saw other threads mention World's Best Cat Litter but I don't think we can get it here in Canada. Any suggestions?

Thanks,
Trish

ps Sorry it's long... apparently when I decide to post I post novels.

You have to switch litters gradually. Start by mixing half and half and slowly reduce the amount of the clay litter. Cats don't like major changes and they hate quick major changes!

Yup, gradual is usally better - in fact, I'd start out with just a tiny bit of the Swheat scoop (which I use, btw) and your normal litter. And then just gradually add more and more.

I actually doubt that clay litter can cause a URI - since URI are viruses, just like our colds are viruses. It might aggravate one.

OTOH, I used clay litter for many years - and never (so far, knock on wood) - had a cat with a URI.

I switched to Swheat Scoop mainly because I just think it's better for them. I tried World's Best, too, but I live in a hot & humid state, and had odor problems with it.

P.S. I don't think you write novels at all.