Question - can it target cards in your hand or only on the field?
Only field.
So it can only be used on creatures/spells? Seems very limited, and should be able to be played on cards in your hand. The mechanism for targeting cards definitely exists (discard).
Creatures/spells? Unless we have completely different definitions of 'hand' and 'field', I think you misunderstood - it targets the player field like Lightning, Blessing, Explosion etc.... You can only target creatures and permanents that were played.
It's not as limited as it looks. Upgrade allows access to better creatures, cards, and abilities - for example it can be run with Angels to turn them into Archangels - a +6 | +1 boost. It can also act as a CC for Growth creatures since their stats reset - this might be necessarily if you need to slow down a Graviton Fire Eater or weaken a Lycanthrope to use other CC on it. Downgrade is more offensively oriented - upgraded cards turn back into their weaker (or unfit for the situartion) unupped versions and permanents will usually get a cost increase in ability use. Both versions can make Sundials/Timed Shields last one more turn as well. (Stolen Permanents with a timer gain +1 to their timer duration).
As for the target in hand, the community seems a bit iffy on coding - but I think if this card comes in the game that should be up to Zanz.