As an Order of the Scribes Wizard, can I cast a Psychic Fireball at Level 7?

Question

According to the Awakened Spellbook feature of the Order of Scribes school of magic presented in Tasha's:

When you cast a wizard spell with a spell slot, you can temporarily replace its damage type with a type that appears in another spell in your spell book, which magically alters the spells formula for this casting only. The latter spell must be of the same level as the spell slot you expend.

So, if my Wizard is 7th level and has Phantasmal Killer not prepared, but in my spellbook, and I cast Fireball using a 4th level spell slot, I can deliver a 9d6 Fireball that deals psychic damage as many times as I have 4th level spell slots available?

Answer 1

This works.

A fireball cast at 4th level is properly a 4th level spell. The rules for 'Casting a Spell at a Higher Level' say:

When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting. For instance, if Umara casts magic missile using one of her 2nd-level slots, that magic missile is 2nd level.

The fireball is 4th level, so you can replace the fire damage with psychic damage if phantasmal killer appears in your spell book. It does not have to be prepared.

Answer 2

Yes

As you correctly cite from the source, the mechanic here is based on comparing the spell slot you're spending with the original spell level of a spell that's in your spellbook. In your case, you can safely cast a fireball that inflicts 9d6 psychic damage if you have Phantasmal Killer in your spellbook.