What did you think of a fat old man playing that blond at the beginning?

My thoughts on the guy in android-drag in the movie? That he very obviously had a hunger for adult hunky males and not little children.
It was very obvious that he was living his dream of being a desired bombshell able to attract really hot hunks.
That being said, there would in such a world where this technology was avaible, of course as a consequence also be paedophiles that would want to act through a child surrogate as a way to meet children.
It would however probably have been made a very difficult thing to do, as there most likely would have existed some type of prevention on whom got to use a child surrogate. Therefore, the usage of child surrogates would probably come with some sort of safety mechanism, perhaps a certain code that were to be carefully memorized to only exist inside the child's head, so that only the child could use it and no one else.

Yeah, "catfishing".

That idea is far from a new concept though. Naming just a couple instances predating this movie...

TOTAL RECALL, (original one with Arnold S.)
Remember the disguise the 'Quaid' character (Arnold) used when first arriving on Mars? It was a 'fat woman', and it developed a glitch, so he was spotted by the 'bad guys' at the entry port. He took off the animated woman's head and threw it to a guard(?). It turned out to be a bomb, exploding to give him cover to escape onto Mars... But, the unstated possibility of the "virtual life" **might have** included a gender swap, since the whole idea was how you wouldn't be yourself in the process, as you would have been if you actually took a vacation trip, where you would still be yourself.

XCHANGE, with one of the Baldwin brothers as the main character.
It was a secondary plot line that a human could swap bodies with a surrogate, or another human, and swapping genders was something of an amusement to a 'select group of clients' using unsanctioned/illegal mind-swap machines. (Want to find out how your lover actually 'feels'? Just temporarily swap bodies with them... or...)
In this movie, the surrogates are a little like the androids/(synthetic humans) from the movie BLADE RUNNER, with an "expiration date" (multiple weeks only in XCHANGE versus years in BLADE RUNNER), and that concept is a little like another Arnold S. movie THE SIXTH DAY where exact clones of a person can be created, but must be destroyed by the sixth day, by law, and lack an expiration date, but have no knowledge they are a clone. There is the risk the clone would want to live, and kill the original human, possibly taking over their life, if they discover they are a clone and will be killed.
Both of the BLADE RUNNER and SIXTH DAY "created humans" are intelligent, but the XCHANGE creations are only empty biologic husks that have the intelligence of the 'user' actually transferred into them, leaving the original owner's body as a mindless husk, but intended to be temporary. Those created husks are the 'worker class' intended to be used for dangerous jobs/occupations (soldier, miner, chemical/nuclear industry, or some similar thing) so that the original intelligence can be transferred back into the human's body, and no human dies in the process. But, there is the possibility the surrogate/husk could be damaged or trapped/isolated so the human's intelligence dies with the surrogate, possibly simply because they cannot get to a transfer machine before the surrogate 'dies'.
In XCHANGE, a 'bad guy' wants to take over the life of the Baldwin character, and kidnaps Baldwin's human body, intending to allow his mind to die in the temporary surrogate as it 'expires', and putting his own mind into the Baldwin character's body.

FREEJACK, throw in a time travel/teleport twist to SURROGATES and XCHANGE, and instead of mechanical or cloned surrogates, think of other human bodies, (this is an older movie that predates the human genome project, Dolly the sheep(clone), and the major advances in robotics).
As a person ages, if they are wealthy enough, they can employ a service to research for a person that has died (typically of an accidental death so their body was still young and healthy), and time-teleporting that now-dead person into its future, just at the instant before their death, for a mind transfer into that "body". Naturally since that person is officially already dead, they would have no rights, but they are still are alive when they arrive in what would be their future. So, their mind needs to be wiped blank before the wealthy client's mind is transferred into the 'jacked body. (Stars Emelio Estevez, and Mick Jagger was a mercenary in this movie. It was pretty good, IMHO.)

Now then, SURROGATES leaves the original human's mind in its own body, but linked to the surrogate mechanism, and has a somewhat unanticipated MATRIX-like risk of the human dying if the avitar dies inside the MATRIX.

Gender swap is not a new concept, but the stories, plots, and 'message' of each is different.