Signs point to... no
It's hard to prove a negative... but I can find no rules that allow "downcasting" spell slots.
The full rules on spell slots are here. There is (very shaky) evidence against downcasting in the Heightening Spells section
Both prepared and spontaneous spellcasters can cast a spell at a higher spell level than that listed for the spell. This is called heightening the spell. A prepared spellcaster can heighten a spell by preparing it in a higher-level slot than its normal spell level, while a spontaneous spellcaster can heighten a spell by casting it using a higher-level spell slot[...]
More importantly, no mention, anywhere, is ever about using spell slots for spells that are not that level, either as written or by using the rules on Heightening. The language from the various spontaneous spellcasters is
You can cast any spell in your spell repertoire by using a spell slot of an appropriate spell level.
Unfortunately, what constitutes "appropriate" is never defined, and the dictionary definition, "suitable or proper in the circumstances" isn't much help either. However, lacking further clarification, "appropriate" would seem to mean that it is the "correct" level, the exact level, spell for that spell slot.