How game-breaking, or special-snowflake-melting, would it be to rule that spontaneous casters of any stripe -- arcane, divine, whatever -- could undercast any numbered spell, and swap out any earlier numbered versions they'd learned, just as psychic types can?
Not game-breaking at all. They're already one caster level behind. This will just give some breathing room to their already existing spell retraining mechanic. I actually hope Occult Adventures will include a rule for that (not a trait, not a feat, just flat out a rule allowing spontaneous casters to do it).