You now cast the strongest spell of its tier. Rather than having to cast Stone 1 when at a lower level and Stone 2 or 3 at a higher level, your "Stone" spell upgrades or downgrades automatically depending on your level so you only need 1 button on your hotbar. It's a quality of life update.
Lots of spells and potencies for BLM have been buffed in damage to account for losing out on Raging Strikes.