That would be correct. But, I said to negate the check. I couldn't find anything in the the rules that said you cannot do that so I made it a house rule. The idea is if someone upgrades a green to a yellow, I cannot flip one to bring it back down and vice versa with my group to me (i know, you would think it would be unsaid.).
It's implied that you can't because only DP effects on upgrading dice pools are discussed. If a player can upgrade the proficiency of their own pool, or upgrade the difficulty of an NPC's pool...and vice versa for the GM. You can't use them to cancel out someone else's previous upgrade.