Yes, it is a party-wide effect, so it only applies to the spell as it's cast. You can swap gear after casting.

Does anyone know roughly about how much % the set will trigger based on 2, 3, 4, and all 5 pieces? I only have 2 +2 pieces but sometimes it triggers often, and sometimes it doesn't trigger at all. I'm going to guess maybe 3, 6, 8, and 10%, respectively, but if anyone has any hardcore testing to share, be my guest!