Generally speaking, I like the way shipyard tooling is set up to be able to multiple similar classes. Several times now, however, I've come upon a situation where it seems like a SY should be able to do a refit, but it isn't. The issue is that a SY can't do an identical upgrade on an electronics or weapons suite, even though the only difference between the classes is the engines or hull material.
Concrete example: A while back, I went from thermal 8 to thermal 11, and redesigned my standard 3HS-24 thermal sensor to a 3-33 sensor. My standard destroyer design (call it Nelson VI) had MHD engines with fuel efficiency 0.7. I introduced a new class (call it Nelson VIb) with E7 engines and 3-33 sensors. My SY was tooled for Nelson VI, but could refit ships to Nelson VIb since the refit price was small (because they had such similar systems.
Then I researched fuel efficiency 0.6, and designed new E6 MHD engines. This led to a new class (call it Nelson VII) with E6 engines and 3-33 sensors. Note that Nelson VII and Nelson VIb are identical designs, except one has E6 MHD engines and one has E7. The weapon and sensor suites are identical. In addition, the refit from Nelson VI to Nelson VIb consists of (figuratively speaking) unbolting a 3-24 sensor and plugging a 3-33 sensor in to replace it. It feels like this should be the same work, whether or not the ship has been up-engined.
This leads to the problematic situation. What I'd like to do is retool my SY to the Nelson VII standard, so that all new construction is up-engined. The problem is, that this prevents me from upgrading the sensor or weapons suites of Nelson VI using that SY, even for refits that don't touch the engines. This isn't a big deal at the moment - the main problem shows up when I introduce the Nelson VIIb or VIIc or VIId classes (with better weapon or sensor suites) - I won't be able to make similar improvements on the Nelson VI set of ships using that SY. And since 6000 HS SY are in short supply, this means that I'll effectively have to freeze the Nelson VI series of hulls at their current weapons/sensor mix.
The downside is that I don't have a good suggestion to solve the problem. The only thing I can think of is to look at the changes being made in a refit, and if the new systems are a subset of the systems on the ship that the SY can build (note the subtle difference here between "is tooled for" and "can build") then it can do the work. So for example, any SY that could build an existing class with 3-33 sensors could perform an upgrade where that is the only change to the class. The determination of what classes can be built by a SY would stay the same.
The thing I like about this idea (if it can be worked) is that breaks some of the coupling between armor type, engines, weapons/sensor suites, and payload type (e.g. cargo hold vs. cryogenics vs. troop transport bay) in the refit process. IRL, it mimics the recommissioning of the New Jersey class BB's in the 80's - the SY in question just needed to be tooled for the weapons/sensor systems (e.g. fitting Phalanx, Harpoon, Tomahawk, etc.) rather than being tooled to build a BB.
John