Your diffuculties arise from the way you talk about the problem.
You ask how you can be sure that a person can change their identity? Your question already presents a mechanism that naturally doesn't work. What is implicit in what you're saying is that changing somehow involves taking something out and putting something completely different in. As in taking out one set of desires and replacing them with some other desires. That is certainly not what happens.
To change some of your desires is to understand them and understand how they arise. That's very related to how you talk to yourself and what expressions you use and where they implicitly lead you. That is the way you speak imposes a certain view of the world, what it consists of and what is possible within it. (That's not to say that you would suddenly be able to fly but there is a whole human domain that you might start discovering from the inside as a human being rather than looking at yourself from the outside as a collection of organs).
You ask how you can be sure that a person can change their identity? Your question already presents a mechanism that naturally doesn't work. What is implicit in what you're saying is that changing somehow involves taking something out and putting something completely different in. As in taking out one set of desires and replacing them with some other desires. That is certainly not what happens.
To change some of your desires is to understand them and understand how they arise. That's very related to how you talk to yourself and what expressions you use and where they implicitly lead you. That is the way you speak imposes a certain view of the world, what it consists of and what is possible within it. (That's not to say that you would suddenly be able to fly but there is a whole human domain that you might start discovering from the inside as a human being rather than looking at yourself from the outside as a collection of organs).