You don't want to do k10 k2tog, because that will span 12 stitches, and 12 doesn't go into 55
The good news is, 55 is the perfect number to work with, because, like 45, it is divisible by 5, so you can still have a 5-pointed star type decrease, which is what the original one gave you.
The original one had 45 sts, and was k7, k2tog. The first decrease spans 9 sts., which, over 45 stitches, will give you 5 sections, and 5 decrease lines. Hope that makes sense.
In order to keep a similar decrease ratio, you want to keep it a 5-point decrease, so....
55 divided by 5 = 11. You want your first decrease row to have decreases spanning 11 stitches. This means k9, k2tog.
This will give you extra decrease rows, but this is GOOD, because your hat is nearly 20% bigger around (18% to be precise), so you also want it to be bigger going up, or it'll be a short hat. In fact, you'll also want to add more rows before you begin decreasing. If your lucky, your existing pattern says knit "this many inches" before decreasing, rather than "this many rows," so you don't have to do any math at all, just knit that many inches. If you have to figure out how many rows, then, let's see.... 18% more rows might be right, but I'm not sure, so do some math and figure out how deep this will be, and hold a ruler up to your head and see if it looks right.
As far as the actual decrease rounds, they should work out perfectly, just decreasing at whatever rate they tell you, but starting with k9, k2tog.
Hope that helps!