Indian soap dramas are different from the usual format of the English dramas/series. Indian soap dramas have 5/6 episodes per week running all year long, spanning across many years.
India's first soap opera was Hum Log, which concluded with 154 episodes. Abhhishekam (Telugu) is the longest running serial in the history of Indian television with 3,100+ episodes as of December 2018 (still running). Yeh Rishta Kya Kehlata Hai is the longest running Hindi serial in the history of Indian television with 2,800+ episodes as of December 2018 (still running)
Content creation for such long running dramas is always bound to run out due to the longevity of the series. Here is where we come to your question -
With Indian serials/dramas (<PERSON> falls in this category) that have a daily episode, the ending is not pre-decided and no definitive climax exists. Only the first few months or the first year the title and the story are the same. The story keeps prolonging and the initial plot/ name of the serial will stop meaning something after a year or so. I'm not bashing Indian serials, but its just how the pattern is. 942
For serials that bring out a daily episode , its difficult to hold on to the initial plot-line. And related to changing the story, unless the viewership drops, the story goes on into unrelated but cliched twists and turns and new plots.
Unfortunately most of the serials start out with a different concept and fall into the only categories that allows for its survival after running out of content - evil family members, maybe evil mother-in-law, stealing family asserts and property, memory loss etc.
The first Indian soap opera, Hum Log, began as a family planning program, and although it quickly turned its focus to entertainment
For example there was this serial called 'Ishq ka rang safed' which initially was about love for a widowed woman and breaking rules, until:
"Ishq Ka Rang Safed (English: White Is The Color Of Love) is an Indian television drama.. The show portrays the story of a young widow, <PERSON>, and a rich boy, <PERSON>, who falls in love with her and goes against the orthodox views of society to marry her. The show was very much accepted by the viewers at an early slot.... But its viewership dropped after the change of the concept..."
So No, "story from start to end of this serial already decided before launching"
Both Yes and No to "changing the story in the mid as per the response of viewers"
Finally I'd like to say that this is not a Indian dramas-bashing answer. Indian dramas cater to people who would like to follow a show everyday without waiting for a week. But the content gets bad because there is only so much you can do. Kudos to anyone who can come up with great content for every single day year after year. 942