Why twin 15-inch turrets cannot become triple 14-inch mounts

Would the Royal Navy have been better replacing twin 15-inch turrets with triple 14-inch turrets?

No; the existing barbette is too small for three 14-inch guns, and the performance advantage would not justify such a conversion.

A barbette for twin 15-inch guns is too small for a triple 14-inch mounting because the reduction in caliber does not free enough space. A twin 16-inch barbette offers a more plausible starting point for three 14-inch guns, though that possibility is qualified. If the Queen Elizabeths had originally received triple 15-inch turrets, a later triple 14-inch conversion would have been easier. There would still be little reason to exchange equal numbers of guns: the newer 14-inch weapon did not offer enough extra range or rate of fire to justify it.