I find it interesting and a testament to Lucille Ball that her 60s sitcom (especially in its later seasons) consistently managed to top so many contemporary sitcoms when they were in their prime such as Gilligan's Island, Bewitched, I Dream of Jeannie, That Girl, Green Acres, Hogan's Heroes, F Troop, Get Smart, The Munsters, The Addams Family, My Favorite Martian, Petticoat Junction, The Monkees, ect. The only ones that I can think of that consistently managed to garner the same ratings (more or less) as her show were The Andy Griffith Show and its spin-off, Gomer Pyle, U.S.M.C. In fact, those two and her show were the three highest-rated of the 1967-1968 season (the finale season of both TLS & TAGS). Its just amazing that Lucille Ball managed to still rule the airwaves given the so many changes that took place with her show.
The changes that took place within The Lucy Show kept things fresh, that's for sure. I tend to think of The Lucy Show of having two different formats and within those formats, two groupings. Season 1 being by itself as it was the only season filmed in black and white and with Vivian Vance and thus could easily be perceived as another season of I Love Lucy sans the boys; Seasons 2-3 together as it showcased Lucy with her two main series co-stars (Vivian Vance and Gale Gordon) and retained the I Love Lucy feel despite being filmed in color. Season 4 by itself with Lucy (sans Viv and no kids) going to Hollywood, meeting celebrities and getting settled in a different setting. It is also the season that changed the relationship between Lucy and Mooney as it established them as boss and employee. That particular relationship would be the driving force from here and on out, thus giving off a different feel to the series. Seasons 5-6 together as the scripts were titled towards Lucy working at the bank and featured guest stars and musical numbers shown more frequently. And I 100% agree that if you compare to season 1 to 6 and visa versa that one might come to conclusion that one was watching two different shows (instead of just one). It's that drastic. The earlier Lucy Carmichael was so much like Lucy Ricardo only to be morphed into a pre-Lucy Carter seasons later.