The public has learned by the start of 1965 that Vivian Vance is leaving The Lucy Show (rumors to that effect had been around since the summer of 1964):

http://news.google.com/newspapers?id=jVMRAAAAIBAJ&sjid=ZpUDAAAAIBAJ&pg=3145,2056486&dq=vivian+vance&hl=en

Lucy goes to New York in the spring of 1965 and tries to persuade Vivian to stay:

http://news.google.com/newspapers?id=8QwwAAAAIBAJ&sjid=HQUEAAAAIBAJ&pg=4794,2177098&dq=vivian+vance&hl=en

Vivian stands firm on her decision to leave the show:

http://news.google.com/newspapers?id=MdkLAAAAIBAJ&sjid=AlcDAAAAIBAJ&pg=3306,4803558&dq=vivian+vance&hl=en

And check the last paragraph in the article from June 1965. Rumor had it that Vivian Vance would have returned to The Lucy Show if Lucy had given her a percentage interest in the series. I so wish Lucy had consented to that. Vivian deserved it!

http://news.google.com/newspapers?id=OuY0AAAAIBAJ&sjid=8nwDAAAAIBAJ&pg=3058,2123704&dq=vivian+vance&hl=en

Lucille Ball says you can't replace Vivian Vance:

http://news.google.com/newspapers?id=QnQ1AAAAIBAJ&sjid=IEYEAAAAIBAJ&pg=4756,2827056&dq=vivian+vance&hl=en

As early as January 1965, rumors swirl that Ann Sothern will replace Vivian Vance on The Lucy Show:

http://news.google.com/newspapers?id=ejEvAAAAIBAJ&sjid=fKoFAAAAIBAJ&pg=763,1759012&dq=vivian+vance&hl=en

http://news.google.com/newspapers?id=STEjAAAAIBAJ&sjid=k9cEAAAAIBAJ&pg=3154,5484683&dq=vivian+vance&hl=en

As early as late May 1965, the public begins learning details about the new format for The Lucy Show: Viv will get married and move out, Chris will go to college in California, "and to be near her, the redhead moves West and puts her son in a military school":

http://news.google.com/newspapers?id=hGcQAAAAIBAJ&sjid=z4sDAAAAIBAJ&pg=1714,3662660&dq=vivian+vance&hl=en

http://news.google.com/newspapers?id=lkUgAAAAIBAJ&sjid=BJsEAAAAIBAJ&pg=5455,3553890&dq=vivian+vance&hl=en

http://news.google.com/newspapers?id=W2wfAAAAIBAJ&sjid=TNUEAAAAIBAJ&pg=5108,2571553&dq=vivian+vance&hl=en

Lucy says she'll miss Vivian Vance, but has signed a number of guest stars to help her launch the new version of her show:

http://news.google.com/newspapers?id=jJwSAAAAIBAJ&sjid=QvcDAAAAIBAJ&pg=1248,4569612&dq=vivian+vance&hl=en

http://news.google.com/newspapers?id=nWcQAAAAIBAJ&sjid=z4sDAAAAIBAJ&pg=999,914388&dq=the+lucy+show&hl=en

The new version of The Lucy Show makes its debut:

http://news.google.com/newspapers?id=mM8nAAAAIBAJ&sjid=d0kDAAAAIBAJ&pg=5019,2702765&dq=the+lucy+show&hl=en

Upon the debut episode of the new format in September 1965, a reviewer writes that the new show is "a slow starter with Vivian Vance absent and definitely missed":

http://news.google.com/newspapers?id=0wYjAAAAIBAJ&sjid=tCcEAAAAIBAJ&pg=3606,2166465&dq=vivian+vance&hl=en

Some critics apparently predicted The Lucy Show would fall in the ratings and "confounding" their expectations, a month into the new season it continues to do well in the Nielsens:

http://news.google.com/newspapers?id=SfIjAAAAIBAJ&sjid=ZQEEAAAAIBAJ&pg=7261,5225257&dq=vivian+vance&hl=en

http://news.google.com/newspapers?id=CJQtAAAAIBAJ&sjid=Yp8FAAAAIBAJ&pg=7030,3096900&dq=the+lucy+show&hl=en

http://news.google.com/newspapers?id=A5AUAAAAIBAJ&sjid=7wQEAAAAIBAJ&pg=4717,5803331&dq=the+lucy+show&hl=en

Now free from The Lucy Show, Vivian Vance happily returns to the stage in late 1965:

http://news.google.com/newspapers?id=atUzAAAAIBAJ&sjid=MOsFAAAAIBAJ&pg=953,1878190&dq=the+lucy+show&hl=en

Lucy's busy career in television continues on without Vivian Vance as her sidekick:

http://news.google.com/newspapers?id=L3E0AAAAIBAJ&sjid=U8kEAAAAIBAJ&pg=840,4853034&dq=vivian+vance&hl=en

It's too bad that since Vivian decided to leave the show Candy Moore, Jimmy Garrett, and Ralph Hart ended up without jobs as well.

Very disappointing but I guess she needed to do what she felt she had to.