is it possible to change from paragraph composer to line composer without text reflow?

No it is not possible.

virender_CTS wrote: Now at correction stage text reflow when there is any changes.

That is correct, and it also is what should happen. Suppose you delete a word and the Paragraph Composer recomposes the entire paragraph, so all spaces are equally divided among the lines. How do you think using the Single Line Composer would improve on that? Spacing out the entire line? What if you insert a word -- should everything on that same line be squashed?

The only difference is that the Paragraph Composer equalizes the entire paragraph, where the Single Line Composer only works 'forwards'. It's precisely because of that the Paragraph Composer is better. You may argue, "yes but it also changes text on lines above a change", and I'll reply "yes -- and so it should, because it equalizes spacing over the entire paragraph."

If you can't stand the flowing text and prefer to have bad spacing instead, add a shift-Return at the end of all text lines above the first line on which you make your changes.

@Virender – hm, changing the composer will force a reflow.

You cannot prevent this. What should be possible is to document the "current state" (numbers of characters per line of text), change the composer and do something with tracking or kerning to get the same amount of characters again (line by line). However, if that will work in every case is uncertain. Other variables could add problems to achieve this. Without knowing the layout, no-one can tell this.

Uwe