I think that Changez's shifting identity does cause tension in the novel. On one level, it causes tension between Changez no longer accepts what was once easily accepted. His colleagues at Underwood- Samson are hostile towards his change. Even Jim, who tries to help Changez at nearly every turn, cannot stand by him fully when the change impacts his work at the firm. Changez's shifting identity causes tension as it marks a change from when he was a "happy go lucky," chap that aimed to please everyone as an outsider would. The change from this to the beard growing, increasingly antagonistic individual helps to bring about tension in Changez and the world around him.
On another level, I think that Changez's shifting identity helps to provoke some level of tension in the reader. The reader finds themselves having to confront some significant issues about fundamentalism in seeing Changez's shift in identity. Perhaps, this shift is understood on the reader's part. An equally valid response would be to reject it, seeing it as being disloyal to a country that gave Changez an opportunity. This contrast in the reader's part is another instance where Changez's shifting identity helps to create tension.