My question is in regards to the capstone feature of draconic sorcerer. One portion of the feature involves being able to not use a spell slot to cast Summon Dragon once per long rest.

Does this free cast lock it at its base level and deny upcasting? Or could I upcast it to 8th level, for example, and then choose to not expend the appropriate spell slot?

Casting a spell without a spell slot can't be be cast Using a Higher-Level Spell Slot.

I had a feeling that was the ruling, I just couldn't find it in the books. Thanks!

Quote from R34LM >>

I had a feeling that was the ruling, I just couldn't find it in the books. Thanks!

See the Spell Description.

Using a Higher-Level Spell Slot. Use the spell slot’s level for the spell’s level in the stat block.

If you are casting the spell without using a Spell Slot, you are inherently not using a Higher-Level Spell Slot.