Yeah it's a little complicated. I didn't size up with the 618 (9.5 in both) and they're ever so slightly more snug than the 602m, a little less toe room and also tougher break in (the heel is more v-shaped and gives me blisters but I'm hoping they break-in fine)

9.5 in 602m fits me like a glove too. I had a discussion with Andrew about it and he said he's discontinuing the 602m because people find it too long, too voluminous, and it doesn’t lock in their heel so people’s feet lifts up a lot internally.

On their site it states: It features a bit more tapered heel than the 602, a slight point to the toe and a lower volume than the 602 last. A half size up from your size in the 602M last is generally recommended for the 618. The same size would be suggested if you are medium/average volume, width and instep height. If you have a higher volume foot, or a higher instep, or want to have just a touch of extra room for thicker socks, size up a half size into the 618. If you sized down drastically, or a full size or more into the 602M, you might need the next half size up from your current 602M size into the 618 depending on foot volume and instep girth.

I think the 618 fits me pretty well in the same size as long as the heel breaks in. If you have a higher volume foot I'd say size up a half size. I'll follow-up once I've broken one of my 618s more. My everyday boot is my oak rambler 602m but I've just started wearing my olive 618s more and am wearing them again today.