This is an interesting question, because I would want to argue that the biggest change that is evident in Eliza by the end of this excellent play is actually internal. It is easy to focus on the success of Higgins's experiment, and the way that he is able to pass Eliza off as a upper-class lady, and certainly we see that Eliza is treated very differently by all concerned compared to when she was a caterwauling cockney flower seller. However, at the same time, the biggest and most enduring change comes with the epiphany that Eliza experiences after her success and the way that she is treated with complete indifference by Higgins, who has given no thought at all to her future. The way that she is treated by Pickering, by contrast, who has always been kind and polite to her from the beginning, even when she was a flower girl, teaches her this important truth:
You see, really and truly, apart from the things anyone can pick up (the dressing and the proper way of speaking, and so on), the difference between a lady and a flower girl is not how she behaves, but how she's treated. I shall always be a flower girl to Professor Higgins, because he always treats me as a flower girl, and always will; but I know I can be a lady to you, because you always treat me as a lady, and always will.
The enduring change that is produced in Eliza is therefore not in her outward appearance, how she looks and speaks, but in the knowledge that the true indication of a person's worth is not to be discovered in such outer trappings.