Back in the day, boat builders made minor changes to boats and named the Mk 1, Mk 2, etc. Often these changes reflected minor performance improvements or ease of use. Things that did not substantially change the basic boat.
Speaking to the Sabre Line, the Sabres 30 MK1 and MK2 have the same hull and deck, with the Mk 2 having a slightly taller rig. Same with the Sabre 28. Later, after the boat industry economic disasters of the 80s, Sabre made substantial improvements in hull and rig on the 36 and 38 and changed the model names to 362 and 382. However, to the uninitiated the boats look very similar.
One of the factors that determine the extent to which a model changes is the cost of building new molds. Molds wear out at around 100 hulls and need to be replaced or significantly refurbished. Builders will try to amortize the cost of the mold and its development over as many boats as practical. On a boat like the Cat 30, keeping the hull the same, but changing the deck and cabin top gives it a new feel and contemporary appearance. It is much less expensive to build just a new deck mold than it is to design and build new hull and deck molds.
When the molds for the Pogo 36 reach the end of their life, Pogo will have to make a decision, keep making the boat, make a MK2 with a few subtle design changes and improvements, or go with an entirely new design.
If I recall correctly, Beneteau recently did this with 36.7s. The market for the boats was getting saturated and the molds a little long in the tooth. Time for a new design.
The main point is that many factors come together to determine what kind of boat a builder will make. Catalina, IP, Tartan continue to build sailboats with a more traditional design. Tartan has tried to meet the more contemporary and racing style with the C & C line. Sabre bailed out of the sailboat business and has been very successful as powerboat company with the Black Cove and Sabre lines.