Yarn help

I am starting a sweater that is knitted top down and is to be done at 16 stitches to 4 inches stockinette. The yarn I have is gauged at 18 sts with size 8 needles. To get the 16 stitches to 4 inches I need to go up to a size 10 needle. That makes the sweater seem way to flimsy. Can I try one size larger and do it in the size 9 needles which gives me roughly 3.5 inches to 16 stitches?
The yarn store said the yarn would be fine, but I’m not thrilled at how it’s looking. The pattern calls for worsted/medium wt, which this yarn is. The yarn store said the gauge would be fine, but I’m not thrilled with how it’s looking using that large of a needle.

Don’t just arbitrarily change needle size AND do the next size larger. First you need to know how many sts per inch you get on which needles so CO about 24 sts and work 2-3" with the 8s, then switch to the 9s and knit another 2-3". Decide which needle makes a fabric you like and how many sts per inch that is. Then divide that into the sts for the next larger size and see if that will give you the right finished measurement.

Since it’s top down though, you can also CO the sts for the next size larger and only do the increase rows you need to get to the size you want. Try it on when the back sts measure about half the finished measurement you want and see if you need to do more inc rounds or rip out a couple - you want the raglan inc lines to almost meet at your undearm.

Thanks for your help. I’ll keep that info in my file for future reference. Decided to try a different pattern. It’s still a top down, but this Anne Norling pattern has instructions for different wt yarns and the gauge is dead on for the cardigan at 18 sts to 4 inches.