interesting idea... but for it to really work, would need to do a few extra things. like have the hp of a monster on your field, effect the amount of damage your opponent can do to your health, or something...
if you could upgrade your abilities, allot of people that have thousands of gold saved up, because they have every card, and keep playing, getting more and more money, would just go in and raise the attacks of key monsters, then lower the cost. so lets say you can upgrade 3 times, each upgrade brings the cost up by 1 quantum. and to downgrade the cost, is by 1 quantum, and you can only do it twice to one creature. well. there is
Pegasus
Card type: Creature
Cost to summon: 4 Light
Stats Damage/Life: 3/2
Ability: Dive 1 Air: Damage dealt for 1 turn is doubled
fully upgraded, and cost downgraded
Cost to summon: 5 Light
Stats Damage/Life: 6/2
Ability: Dive 1 Air: Damage dealt for 1 turn is doubled
i love to equip
Blessing
Cost to Use: 3 Light
Use: The target creature gains +3/+3
i can sometimes get 3 of these put on my Pegasus, wich when its ability gets used it would be doing 30 damage a turn upgraded like this, and only cost 5 light to get him out, 9 light to equip the cards, and 1 air every turn. that creature is taking a 3rd of someones life each turn. and with the limited array of cards, hard to get rid of if you dont have the right deck. on normal fights having 4 of those guys out, 1 has 3 bonuses, and a golden dragon, i still will sometimes end up with 20+ light quantums.
being able to upgrade cards is a great idea, but because of how hard it is to make gold in this game, it would mean even more grinding for new players to do, so they can get a deck, and get key creatures powered up so they can compete with the people who have been playing for a long time. and upgraded the creatures they wanted right off as soon as this was put into the game.