Trump has been losing support in huge numbers for months. He has a very low number of people behind him at this point, and those that are seem to really like his rude, obscene, and vile remarks and ideas. With him backing down on immigration, he is sure to upset some of his current supporters.
When a political party or in this case one running for office softens his stance it is to his best interest to gaining more support, not to lose it. Those who are politics know exactly what to say and how to change it to bring in the results needed. Donald has advisers and has been in business long enough to know how to smooth talk a person, but it know seeing how difficult it is to move the masses.
Although Trump will lose a very small amount of supporters, most of his supporters will just find an excuse why he doesn't really mean what he now says about immigration. "The media is twisting his words" or "He's just saying that to get elected" are sure to be favorite replies to Trump's flip-flopping.
Presidential candidates are notorious for changing their views to match popular opinion and changing political climate. Many voters have only one key issue that they care deeply about. By attempting to expand his appeal to reach people who want a softer immigration policy, Trump does not risk losing his base supporters.