My reading of the rules is that a lower level spell (say 1st level magic missile) upcast to a higher level slot (say 7th level) becomes that level of spell - the magic missile is treated as a 7th level spell in all respects - Including affecting a monster that's not affected by 6th and lower level spells.
Support (from the PHB):
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. Effectively, the spell expands to fill the slot it is put into.
I ask because I caught a live stream of a high level game, where the DM ruled the exact opposite (a magic missile upcast to 7th level had no effect on a monster immune to 6th and lower level spells). Pretty sure that was incorrect, but am I missing something?
Support (from the PHB):
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. Effectively, the spell expands to fill the slot it is put into.
I ask because I caught a live stream of a high level game, where the DM ruled the exact opposite (a magic missile upcast to 7th level had no effect on a monster immune to 6th and lower level spells). Pretty sure that was incorrect, but am I missing something?