When you change your purpose statement, you are proposing an entire new line of research. This can make a big difference even as you are doing research on the same original problem.
On a previous question of yours, I gave the example of a study on negative advertising in political campaigns. The general statement of the problem was that negative campaign ads exist and that they are said to have bad effects on voters. The purpose statement said that we would try to research the effects of such ads on voter attitudes towards the political process.
But imagine if we changed the purpose statement a bit. We would still be studying negative campaign ads, but now we would look at whether they work or not. We would change our purpose statement to say that we will look at the impact that the negative ads have on voters' attitudes towards the target of the ads. We could also look at the impact on attitudes towards the maker of the ad.
In this way, we would still have the same problem statement, but we would now have a purpose statement that completely changed the focus of the study.