spell slot table 5e

The Paladin Table above highlights the number of spell slots available at each level. The paladin must select a number of spells equal to their charisma modifier plus half of their paladin level, rounded down.

In Tier 4, (Levels 17-20), the first level gains the 9th level spell slot, but then all the other levels give lower level spell slots.

Now that the Sorcerer has spell slots of differing levels, they can choose to cast a previously level 1 spell as a level 2 spell using a level 2 spell slot.