Help with Wild Tables

Is there anyway to set a flag or event that permanently alters the wild Pokemon of an area?

Rather than script out a Roaming feature for the legendary dogs, like Lati@s in Emerald, apply Suicune to Route 120's wild Pokemon table at 1% encounter rate, and then once your Pokédex has had Suicune added to it changes the table permanently so that a different Pokemon such as Zigzagoon permanently occupies that slot?

Thanks would help a lot if someone knew anything about this.

Also if anyone can help, I'm really just trying to get the three dogs, Suicune, Entei, and Raikou to roam in Emerald after the Kecleon/Steven cutscene on route 120.