R-class boiler orders and the Renown design lineage

Were the R-class boilers chosen because the Royal Navy already had them in stock, and could they simply have been upgraded later?

No. The boilers had to be ordered, and a more advanced battleship program would probably also have given the Renown class a more advanced design basis.

A viewer suggests that the R class may have used old boiler technology because suitable units were already in stock or production. The answer rejects that explanation because the boilers still had to be ordered. It then follows the design consequence forward: if Britain had built additional Queen Elizabeths, or made the R class an improved Queen Elizabeth derivative, the Renown-class battlecruisers would likely have evolved from that newer hull and machinery baseline. Procurement timing and inherited design work matter across successive classes, even when the original economy measure appears confined to one battleship order.