I first saw this film at the age of 12, in the age of Dialing for Dollars (Janis Joplin, anyone?) Strangely enough, "Elvis Week" played before Elvis was dead. The Beatles were the week before. I fell in love with both.
I am now pushing 50 and I still love my Beatles and Elvis movies. I must have seen "Change of Habit" 50 times since I was a kid, and the more I see it, the better I like it. It is true some parts are ridiculous, including my pet beef that the nuns are wearing make-up, specifically eye-shadow, in the convent, prior to being "released" into the general population. I consider criticisms such as this "chump change" -- the kind of thing I can accept as long as the "premise" of the movie is reliable.
Serious critics can pick apart many "credibility" moments in this movie. Much might appear "dated." But for me, I look for the "soul" of the movie; the serious intent of the characters -- and the actors behind the characters -- who put their soul into this movie to put forth ideals in ways they were not allowed in their mainstream professional lives.
Elvis (privately) rated this one of the best movies he'd ever done, but, because it was his last movie and was not backed by a soundtrack, was considered by Col. Parker a failure, and therefore did not "exist" as a record of accomplishment financially or through social impact. People who "study" Elvis know he never cared about the "bottom line" dollar-wise; he always cared about people. In this movie, it shows.
Lastly, and this may be a "technical" point, but this is the only movie I've ever seen that even remotely deals with what was called the Ecumenical Movement. I am not Catholic, or religious, but it seems to be that this movie takes on even a sliver of social and human rights issues that were taking place in the late 1960s that really weren't addressed at all until prior to the turn of the century, and is poised to be an issue under the current Papal administration, as I write this.
This movie is not for everybody. Some people would say that "only people who love Elvis would care about this movie." I can honestly say I never cared about Elvis until I saw this movie.