If you feel like your hoop is too big for you now, then by all means move down a size! But keep the big one, it'll still be useful when learning advanced tricks like shoulder passes and horizontal elbow passes. Everyone moves down hoop sizes at a different pace. Before you get a new hoop, I'd measure the diameter of your current hoop so that you know what size it is. From the outside edge of the tubing to the opposite outside edge is called the OD measurement (outer diameter) and from my experience that's how a lot of hoop makers measure hoop sizes. I'd say it's pretty safe to move down in size two inches at a time, but it's totally up to you. Honestly, I moved from a 38" to a 33" in about a month's time. 33" only because I got tired of buying $30 hoops so I started buying the kids' hoops from WalMart (the smallest ones are roughly 33" OD) and gaff taping them. I like doing this because they're only like $7 each so if you don't like the size or if anything bad happens to them (like lost or stolen), oh well you didn't invest very much money in it. And you can always hold on to it until you do want that size. (I had mine for about a year before I actually wanted to use it for hoop dancing). I have to gaff tape all of my kids' hoops though, because they're not very stable. I've had so many naked hoops from the grocery store fly apart on me while leg hooping that it's maddening! Transitioning to a smaller size is always awkward until you find the size that's right for you and you get used to hooping with it... Since I've never seen you hoop, I obviously can't tell you what's best for you. But since I wish this DIDN'T take me like 3 months to figure out I'll share it with you anyway, after a certain point of down-sizing it may not actually be a smaller hoop diameter that you want, but actually a smaller gauge tubing. I was very frustrated with a hoop that I had literally just bought (not from WalMart) because despite it's size, I felt like I couldn't work with it at all. Then I realized that it's weight was a little heavier than what I wanted and that it was harder to keep a grip on because the tubing was so big around. Since then I've found that like 3/4" OD tubing is a really nice guage!
Hopefully at least some of this is helpful to you! Good luck and happy hooping!