The Identity Matrix

Victor Gonser, Ph. D., a middle-aged college professor, is backpacking in Alaska when he finds his consciousness inexplicably transferred into the body of a 13 year old Tlingit Indian girl. Later he swaps bodies with a stunningly beautiful Canadian college student named Dorian "Dory" Tomlinson. Taken into custody by a secret U.S. government agency, Victor learns that the Earth has become the battlefield in a skirmish between two highly advanced and implacably opposed interstellar cultures who view human beings as little more than animals and use body-swapping as an infiltration tactic. Worse, the aliens have killed his old body. There could be no going back.

Permanently cut off from their old lives, Victor and Dory have little choice but to volunteer as both researchers and test subjects at the Identity Matrix Center, a secret underground facility outside Los Vegas where a secret government task force is engaged in desperate efforts to unravel the alien's body-swapping technology and determine which of the two alien sides offers the best chance for humanity's survival, all while "Vickie" learns the ins and outs of being a woman. But when Dory stumbles across evidence that IMC may be playing a deeper game things suddenly take a more sinister turn, because while IMC's understanding of Identity Matrices may not yet be advanced enough to transfer minds between bodies it's certainly advanced enough to make two troublesome girls disappear by giving them new memories and personalities. One false move later and Dory and Victor's personalities are replaced by those of Delores Eagle Feather (a Navajo orphan) and Misty Ann Carpenter, an extraordinarily enthusiastic (and extraordinarily well-endowed) stripper and prostitute.

That's when things really start to get interesting...

One of the best novels by the speculative fiction writer Jack Chalker, The Identity Matrix was written early in his career when Chalker was better known more for his plots than for Author Appeal and remains one of the better examples of the "Earth as a battlefield between two alien cultures" subgenre. Out of print but well worth reading if you can find a copy.

Tropes used in The Identity Matrix include:

Author Appeal: gender benders resulting in improbably busty heroines...and ferryboats, believe it or not.
The Atoner: Dan Pauley, the alien infiltrator, which provides a psychological lever for the humans
Attention Whore: Misty, in ALL senses of both words
Big Breasts, Big Deal: Misty is a classic example of Type 4.
Bi the Way: Both heroines, making this both a fairly early mainstream example and relatively Fair for Its Day.
Body Surf: By the truckload!
Brainless Beauty: Played straight with the new personality designed for Misty but averted later after they restore her stolen memories. After her final "restoration" Misty describes herself as "...a stripper and prostitute who could discuss Von Clauswitz, A.J.P. Taylor, and the fine points of Jungian psychology before going to bed with you."
Buxom Is Better: played straight but lampshaded to show what rat bastards the IMC conspirators are.
Different for Girls: played straight twice from opposite directions: once when Victor has to deal with his Gender Bender and again from the other side when Victor's old male memories get reloaded on top of Misty's thoroughly female personality.
Earth Is a Battlefield: for a covert side-skirmish in a galactic war that few humans even know about.
The Faceless: Harry Parch. No one knows what he really looks like, he's always in disguise.
Fountain of Youth: 35 to 19 in Victor's case, 19 to 13 in Dory's.
Gender Bender: Chalker, natch, but this story was actually written before that was the first thing that came to mind at the mention of Chalker's name.
Government Conspiracy: the IMC
Grand Theft Me: The alien's method of acquiring new bodies. Then they kill their victims to cover their tracks.
Happiness in Slavery: Misty actually likes being a nymphomaniac prostitute because she's finally getting the attention Victor always craved.
Harsher in Hindsight: the novel's depiction of relatively carefree sex workers viewed from a post-AIDS perspective.
Hoist By His/Her Own Petard: The research Victor/Misty does on Dan Pauley is used to reprogram her mind in ways that will punch his buttons.
I Did What I Had to Do: both the alien Dan Pauley and government agent Harry Parch use this to justify their actions, and both are defensive about it when pressed.
La Résistance: The civilian scientists within IMC subverted, it's all part of the plan.
The Masquerade: Boy Howdy!
The Mind Is a Plaything of the Body: Inverted. The mind may not be the plaything of the body but reverse can be the case once you've figured out how to adjust it in ways that affect hormone production in order to do things like produce bigger muscles (or in Misty's case bigger boobs).
Morality Pet: The heroine serves as one for "Dan Pauley", the captured alien commando.
Really Gets Around: Misty before her restoration -- and after, for that matter.
Sliding Scale of Idealism vs. Cynicism: Played with: The heroines appear to win out over both the aliens and the Government Conspiracy only to have the epilogue reveal that they were never anything more than pawns in a deeper Plan in the first place -- on the part of the person they thought was the head of La Résistance, no less. Then comes the postscript to the epilogue, which implies that at least one alien figured out the plan but played along for idealistic reasons of his own.
Well-Intentioned Extremist: Parch. He does some awful things, but his motives (saving earth from the aliens) are unimpeachable.