If you change tasks at a shipyard (from "add a slip" to "retool", for example), is the work lost? If I'm 98% of the way to adding a new slip, and accidentally shift to "retool", do I begin again at 0% if I go back to adding a slip?
If so, this seems like something which might be changed. Logically, there's a slip 98% of the way built, consuming a lot of resources; it makes perfect sense I can't use it until it's finished, but it seems odd that I can't go back to it. In the "reality" of the game, it's there. This is how research already works -- if you can cancel a research project, you don't lose the points invested in it. (Presumably reflecting experiments, prototypes, journal articles, etc.)
I can see this idea NOT being applicable for retooling, though... it doesn't "make sense" to have, say, a dozen "retoolings" at 98% done on the same shipyard, though I can see a retooling being stopped, capacity being added, and then THAT retooling continuing... but chancing to a new retooling should start at 0.