So is this 10 for 5 (unupped) and 8 for 7 (upped) or does the cost count for the requirement shown in the text?
If you're interested in keeping it a spell, lower cost to 2 .You could also try making it a permanent and have the conversion be an ability.
i was expecting this to allow a creature to gain the ability of grow, which could be interesting if you make it a duo element and not mono