I think Mary's transformation has been steady, in the sense that it didn't happen overnight and relatively natural. Sure, she's lost some weight, but Mary was not overweight to begin with and she has not lost weight to the point where she looks sickly, so it hasn't been a very dramatic, unhealthy weight loss. I think her figure looks healthy and athletic - we all know how much she loves sports. There have been some other transformations in terms of weight that do not look like natural weight loss (I won't name any names, but I'm sure some people can think of a few names), but Mary isn't one of them.
Yes, her style has changed a lot and she looks much more polished and refined, but being polished and refined helps her represent her new adopted country - who wants a princess who doesn't look like she cares enough about her role to dress well? I equate this to any other royal, or even CEOs of companies, Presidents/Prime Ministers of countries, who represent something more than themselves and subsequently dress well in order to show respect for what they represent. As a Canadian, when I see Queen Elizabeth II, who is the Queen of Canada as well as all the other commonwealth countries, I appreciate seeing her dressed well, even in her older age, as it shows that she respects her position and the countries she represents. Mary, as a younger princess, of course dresses a little more in style than the older royals do, but she always dresses well, with polish and class, which to me shows that she cares about the way she represents her country and the position she holds.
What some people feel about her work aside from how she looks is a topic for a different thread altogether, but I think Mary's transformation has been natural and appropriate, and see no issues with it.
Whether Mary has lost a bit of her identity is difficult to judge from pictures and footage of her in her public role. As a Crown Princess, she is representing Denmark and the royal family, not herself. How she is in private is a much better judge of whether she's lost who she is... and we will probably never know this unless we know Mary herself. I have a suspicion though that she is still, more or less, the same Mary she was then in terms of character and personality, and I'm sure her family in Australia would probably agree. Yes, she is much more disciplined and professional (for lack of a better word) in her public role, and while I am sure she is still respectful of her position in her private life, I suspect she is more loose, fun and more "Mary from the suburbs" (to quote above) when she is not on her official duties, like when she is with Frederik, Christian, her family, friends, etc.. Of course, we can't know this... so I wouldn't spend too much time speculating on whether or not Mary is the same person on the inside as she was when she met Frederik.