I did as you mentioned and most of those formatting disappeared. Thanks
for
that. Now I want to change the style of a paragraph let say its style is
Arial (complex) Arial 12 bold (and this has no stlye name. Is it possible
to
assign a name at this stage?) to another stlyle let say Arial (complex)
Arial
12 (this one also has no style name). I put the curser in desired
paragraph
and select the desired style from task pan (in far right). I expect to see
the change but nothing happens. Am I doing something wrong or there is
something more I have to consider.

If some kind of manual formatting (font, size, space before, whatever) is
applied on top of the style, changing that formatting in the style will no
longer affect the text formatting.
Manual formatting beats styles.

When you unchecked "Keep track of formatting", you just hid the applied
manual formatting, but it is still there.

You'll have to clean it up, or start formatting from scratch.

There are some tools that promise to do (some of) this automatically such as
DocTidy, but I have limited experience with them, and the success likely
depends on whether the (manual) formatting is somewhat consistent to start
with.

Regards,
Klaus