Could I get a verify that, as implied, retooling to build a 'slightly better' version of a ship ("We swapped out the old Mark II missile launchers for the new Mark IIIs") will take a lot less time than retooling to build a completely different ship of the same, or close, tonnage? That is, if Ship A uses 3 type-4 ion drives, and Ship B used 3 type-4 ion drives, the part of the shipyard that assembles and installs type-4 ion drives doesn't need to be retooled and doesn't count, or counts less, against the cost of retooling? Or, in other other words, is retooling 100% based on the hull size of the ship, or is it based on how many components the ship shares with the prior ship?
This is pretty important, as it could effect which yards I retool, and why. I was going on the assumption that it took just as long to retool from the Exeter Destroyer to the Exeter-A Destroyer as it did from the Exeter Destroyer to the Franklin Geo Surveyor, if both ships were the same overall tonnage, so it didn't matter what a shipyard was previously making.
I'm a little rusty, but if I remember correctly retooling is only based on the cost of the ship.
However a shipyard tooled to build class a particular class of ship can also build similar ships without modification. Although I'm not sure precisely how this level of "similar" is calculated, it's based on the cost of refitting a ship from one class to another, which is based on how many components are different and how expensive those components are. There may also be some factors for how similar the ships are in tonnage.
This produces some unrealistic constraints, but as long as you know what they are the system works alright. For example if your Exeter-A Destroyer is the Exeter Destroyer with a new sensor and a few more Magazines you will probably be able to build the Exeter-A Destroyer at Shipyards tooled for Exeters, and vice-versa.
On the other hand if the Exeter-A replaced all of the Exeter's engines and turrets with new ones of updated technology but the same sizes, then they would become too different to share shipyards because the cost would be so different, even though in "real life" it seems like that wouldn't happen. If you had the same number and same sizes of components then it should be completely straightforward to continue using the same installation processes for the ship, only the production of those components would change.