Actually Brad Kern's full influence on Charmed began with Season 4 after Connie Burge was let go.
I think S4 and S5 also showed more of The WB's influence - S4 when they wanted it to replace Buffy when Buffy jumped ship to the UPN and S5 when they started trying to use sex on Charmed to save their dying network.
Using various IMDb pages, here are the writers from the first five seasons, sorted two different ways. First, all of the writers in the order that they appeared, including the first episode they wrote:
Constance M. Burge (1.01: Something Wicca This Way Comes)
Brad Kern (1.02: I've Got You Under My Skin)
Zack Estrin (1.03: Thank You for Not Morphing)
Chris Levinson (1.03: Thank You for Not Morphing)
Javier Grillo-Marxuach (1.04: Dead Man Dating)
Greg Elliot (1.06: Wedding from Hell)
Michael Perricone (1.06: Wedding from Hell)
Edithe Swensen (1.07: The Fourth Sister)
Sheryl J. Anderson (1.09: The Witch is Back)
Tony Blake (1.13: From Fear to Eternity)
Paul Jackson (1.13: From Fear to Eternity)
David Simkins (2.04: The Devil's Music)
Vivien Mayhew (2.06: That Old Black Magic)
Valerie Mayhew (2.06: That Old Black Magic)
Michael Gleason (2.14: Pardon My Past)
Krista Vernoff (3.03: Once Upon a Time)
William Schmidt (3.05: Sight Unseen)
Daniel Cerone (3.06: Primrose Empath)
Monica Breen (3.07: Power Outage)
Alison Schapker (3.07: Power Outage)
Peter Hume (3.08: Sleuthing with the Enemy)
Neil Scovell (3.11: Blinded by the Whitelighter)
Michael Gleason (4.11: Trial by Magic)
Curtis Kheel (4.18: Bite Me)
Mark Wilding (5.5: Witches in Tights)
Laurie Parres (5.6: The Eyes Have It)
Henry Alonso Myers (5.7: Sympathy for the Demon)
Andrea Stevens (5.19: Nymphs Just Wanna Have Fun)
Doug Jones (5.19: Nymphs Just Wanna Have Fun)
(I'm sure you've noticed that some episodes had more than one writer given credit.
In this listing, the writers are sorted by total episodes written that season, then by the previous order)
SEASON ONE:
Brad Kern (5 Episodes)
Constance M. Burge: (4 Episodes)
Sheryl J. Anderson (4 Episodes)
Zack Estrin (3 Episodes)
Chris Levinson (3 Episodes)
Javier Grillo-Marxuach (3 Episodes)
Edithe Swensen (3 Episodes)
Greg Elliot (2 Episodes)
Michael Perricone (2 Episodes)
Tony Blake (2 Episodes)
Paul Jackson (2 Episodes)
SEASON TWO:
Zack Estrin (4 Episodes)
Chris Levinson (4 Episodes)
Sheryl J. Anderson (4 Episodes)
David Simkins (4 Episodes)
Brad Kern (3 Episodes)
Javier Grillo-Marxuach (3 Episodes)
Vivien Mayhew (2 Episodes)
Valerie Mayhew (2 Episodes)
Constance M. Burge (1 Episode)
Michael Gleason (1 Episode)
SEASON THREE:
Sheryl J. Anderson (4 Episodes)
Zack Estrin (3 Episodes)
Chris Levinson (3 Episodes)
Daniel Cerone (3 Episodes)
Krista Vernoff (3 Episodes)
Monica Breen (3 Episodes)
Alison Schapker (3 Episodes)
Brad Kern (2 Episodes)
Neil Scovell (2 Episodes)
Peter Hume (2 Episodes)
William Schmidt (2 Episodes)
Michael Gleason (1 Episode)
SEASON FOUR:
Brad Kern (4 Episodes)
Daniel Cerone (4 Episodes)
Monica Breen (4 Episodes)
Alison Schapker (4 Episodes)
Curtis Kheel (3 episodes)
Neil Scovell (2 Episodes
SEASON FIVE:
Krista Vernoff (5 episodes)
Daniel Cerone (4 Episodes)
Monica Breen (4 episodes)
Alison Schapker (4 Episodes)
Curtis Kheel (4 Episodes)
Brad Kern (3 Episodes)
Henry Alonso Myers (3 Episodes)
Neil Scovell (1 Episode)
Mark Wilding (1 Episode)
Laurie Parres (1 Episode)
Andrea Stevens (1 Episode)
Doug Jones (1 Episode)
So I'll let each of you decided when the change in writers made the difference.
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