Hello, first off, welcome to the forums. Just to let you know, your first few posts have to be approved by a admin or moderator. I saw you posted this twice, probably thinking it didn’t post. I approved this one and deleted the other.
As for your topic, season three was a bit of a change as Leonard and Penny start dating. Some of the shows were going to be about that, this might be the change you mentioned. The addition of Bernadette and Amy were major turning points, no doubt.
While their introduction was in season three, they were added to the main cast in season four. So the major turning points would be, in season three, episode 5, The Creepy Candy Coating Corollary for Bernadette’s introduction and episode 23, The Lunar Excitation, for Amy’s introduction. For their addition to main cast in season four, episode 4, The Hot Troll Deviation for Melissa’s addition and episode 8, The 21 Second Excitation, for Mayim’s.
These were major changes as it gave two other guys ( besides Leonard) regular interaction with other women. It also opened up storylines with just the women and allowed the show to show more of Penny’s life. But, it did change the show to more relationship based. Now, relationships were always there before, and the geekiness was there after, but the relationships came to the forefront and the geekiness fell more to the background.
Finally, while you are correct, this subject has been discussed, it’s new to you, so feel free to discuss it. In those discussions, there were those who disliked the changes, and felt the first three or four seasons were the best. They didn’t like the more relationship based episodes. As these became more frequent, those folks drifted away from the show. Others liked the change and still others just went with the show where ever it went. Everyone was drawn to the show for their own reasons and liked what they liked.