This is a story that I read in a sci-fi magazine in the mid-to-late 1980s, but it may have been published up to 15 years earlier, since I was going through several years' worth of issues at the time. I'd guess most likely a late 60s to mid 70s original publication.
I really only remember the premise of the story and not the main plot. The protagonist wakes up believing himself to be an alien who has somehow transferred his mind to a human host. He does some quick studying of the local culture and decides he needs to make some "improvements" to the host body (exercise, better clothes, contact lenses, etc.). I think maybe he also quits his job or drops out of college or something similar.
Over the course of the next several weeks, I think he either starts to remember a mission he was supposed to be doing, or he notices the activity of other covert aliens on Earth and decides they must be stopped. He then tries to derail these other aliens' plot. I believe he is successful, to everyone's surprise, because, at some point, it's disclosed that the protagonist is a normal (if nerdy) person who has been hypnotized into thinking he is an alien, as a sort of... vacation? self-improvement program? and everyone knows that aliens aren't real.
I'm pretty sure that he had undergone this hypnosis willingly, but there may have been some concern that he was taking things too far and was not willing to return to his normal personality before completing his mission.
I don't really recall whether the other aliens were real or just part of his hypnosis trip. In the end, he does return to his normal personality, but is a very different person due to his experiences as an "alien."
The only detail I remember other than the "makeover" was that the protagonist ran into some acquaintances who recognized him but whom he had forgotten. They may have known his from his prior work or school life. I think they were impressed by the change but confused or put off by being brushed off. Maybe one decides to help him with his current efforts?
Pretty spotty, I know, but I've had luck with similarly warped recollections here before. Anyone recognize this?