Wordsmith

You attempt to make a minor change to a spell you can see being cast within 60 feet of you. If the spell is of 3rd level or lower, you succeed. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level.

If you succeed, you may make a minor change to the title of the spell the creature is casting. This change can, for example, be another word that rhymes, or a substitution of a few letters, but it is ultimately up to the DM. The effect of the spell then changes to something fitting of the new title, as the DM describes. Usually it is a harmless or silly minor effect, or sometimes the spell fails entirely if it is nonsense, but the spell can potentially create an effect up to the power level of the original.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, it succeeds if the interrupted spell has a level is less than or equal to the level of the spell slot you used.