It's only natural to upgrade any kind of long lived ship. This usually involves outward appearance changes. Then they have to be identified by time periods. It happens with real aircraft to. We upgraded 70 Sea King/ and HH3's Helicopters from 3 bladed rotors to an elastomeric rotor head which held five blades, way more powerful engines. The craft were stripped down the the frame, and even the framing taken apart. In the end, it was cheaper than starting a new production line, and increased the lifespan on those helicopters by 6o years. The HH3 is still in use and many countries produce and use these Helo's under license.
Progressive improvement on a solid design makes sense, and adds realism to a story line. The HH3 (on top) has a rear door access point, The Sea King (H-3) below is amphibious and can land and float in some pretty rough seas. While officially retired, both are still in use, and their retirement goes slowly, many being transferred to the Coast Guard.