LilyMods

By this I mean, how do I make a new perk take effect over an old perk instead of co-exist along side it. As an example. Bullseye on the Archery tree has 15% chance for 10seconds of paralysis. If I was to make a Rank 2 that had 20% chance at 15seconds of paralysis. How do I make it override the last perk, instead of it now making my game have two simultaneously active % chances; at doing two separately defined durations? Essentially giving me a 35% chance that I do not want.