Spellcaster that prepare their spells (wizards, paladins, rangers, druids, clerics) can always use a higher level spell slot to prepare a lower level spell, even without the Heighten Spell metamagic feat. The spell acts as normal for its level though (i.e. a magic missile prepared in a third level slot without the Heighten Spell feat still counts as a 1st level spell). A spellcaster can do this even if he wouldn't normally be able to cast spells of that level because of a low ability score (i.e. a 10th level Paladin with a Wisdom score of 11 could prepare a 1st level spell in his 2nd level spell slot). Spontaneous casters (sorcerers and bards) do not get this luxury without the Heighten Spell feat.