Forum:Venom Swamp on ATK boosted monsters

What happens when I have "Solidarity" and "Venom Swamp" on the Field? For example, will my ATK-Boosted non-Venom monster still acquire Venom Counters and weaken until they have 0 ATK (and die), or is the effect retroactive (meaning that the monster must have at least 800 ATK at all times because of the Spell)?