Thanks. The "bio" said she went in for a major retrofit in 1989. I figured that was to make her "improved". Saw the picture after the post.
There are three models, or 'Flights' to the 688-class. Flight I is the compromise design. In Rickover's quest for 30+ knots, the reactor/engine came in about 100 tons overweight. Some of that came out of making the hull a little thinner (call it 2/3rds of the 637 class) and dropping the Vertical Launch System (VLS). Without the VLS, the center of balance on a 688 is a little off, that's why a Flight I usually rides 'bow up' on the surface. Flight II boats are closer to the oiginal design, and have a better sonar system (Q5-C when I was in, probably Q5-D, or perhaps even BSY-1 now) and they have a different reactor, too. That freed up enough weight to put the VLS back in. I was on one of the early VLS boats. Flight III or 'Improved' boats are almost totally redesigned. VLS, an even better reactor, the BSY-1 combat control system, and a redesigned sail for under-ice operations, including moving the fairwater planes to the bow and making them retractable. The hull may even be thinker / made of different material, I'm not sure.