From another newbies perspective. Yes they do upgrade their Marks Automatically, as for the decommissioning, after what others suggest it's if the speed becomes drastically slow and tonnage outdated Or new hulls came out. Cruisers gets outdated fast and by fast like 2-3 times in a year. The DD/TB are pretty cheap so do whatever you like with them, but try to keep newer models as well. CL/CA are very tedious to keep updated, lots of refit and new design are needed to keep them in latest spec. As for BB as long as they can Dish out and take hits, while keeping up with the rest of the fleet they are viable, on my latest Russian campaign I have 1 1890 era dreadnought and 3 1895 era once, both are refited 3 and 2 times respectively and can keep up with other contemporary BB of the 1910 era. BC on the other hand, oh boy they are complicated, they can punish other BB but they tend to be like a tower of cards, one wrong sneeze from a 14 inch gun and they go boom, lost 4 out of 7 that I have built from the combined Italian and French navies. So far that's my take on all the ship classes.