What if every German battleship, including pre-dreadnoughts and ironclads, became a Bayern and every battlecruiser an Ersatz Yorck in 1912, with infrastructure upgraded to match?
Short answerBritain would respond with maximum fast-battleship construction and likely 16.5-inch guns, forcing Germany into an even more expensive arms race.
NotesTaking the transformation to occur on 1 January 1912 puts it alongside British orders for the Queen Elizabeth class. A German fleet of 15-inch-gun ships is expected to push Britain toward the 16.5-inch/45 gun then under development and roughly 28-knot fast battleships. That proposed speed would exceed the 27 knots cited for Ersatz Yorck. British yards capable of building battleships would receive enough work to prevent their failure, with possible orders of eight or even twelve ships in each of 1912 and 1913. These are speculative responses to the scenario. The question grants Germany supporting infrastructure upgrades, but larger ships would still need a larger maintenance budget and compete with army spending. Britain's eventual replacement fleet and closer surveillance of Germany would complicate the initial advantage.