Effects that modify a creature's power and/or toughness without setting it will apply to the affected creatures no matter when they started to take effect. The same is true for counters that change a creature's power and/or toughness.

Archon of the Wild Rose's ability overwrites all previous effects that set the affected creatures' power and/or toughness to specific values. Other effects that set these characteristics to specific values that start to apply after Archon of the Wild Rose enters the battlefield will overwrite this effect.