A spellcaster always has the option to fill a higher-level spell slot with a lower-level spell.
As written but my question is does it increase the DC of the spell? it doesn't state anywhere this if it does or doesn't unless "with a lower level spell" is the implied indication that it doesn't but yeah...not sure anyone know?
Not unless you use heighten spell.
Benefit: A heightened spell has a higher spell level than normal (up to a maximum of 9th level). Unlike other metamagic feats, Heighten Spell actually increases the effective level of the spell that it modifies. All effects dependent on spell level (such as saving throw DCs and ability to penetrate a lesser globe of invulnerability) are calculated according to the heightened level.
It doesn't.
That is why the Heigghten spell metamagic feat exists.
thank you!
No it doesn't. you are using the slot to prepare the lower level spell in as it would be in that lower level spell slot. Kind of like putting 250ml of liquid in a 500ml bottle. You need heighten spell to increase the DC of the spell. Rather than just acting as a space to store the spell heighten spell actually allows you to treat it as a spell of the new higher level.
TL:DR a 1st level spell stored in a 9th level spell slot is still a 1st level spell. A 1st level spell heightened to 9th level is a 9th level spell.