How challenging the major is will depend on how challenging the department is at your school and what classes you choose to take, so unfortunately i can't help you with that one...
onto the other questions: i think psych is a great choice for people interested in medicine or the medical field. it's a field that focuses on people and treating problems, though how "science-y" the problems are depends on what type of psych you do. there are a many fields of psychology i would be interested in entering if i didn't end up in med school for whatever reason. personally, i'm into neuroscience and neuropsych, so i would most likely become a neuropsychologist. if i didn't do that, i would probably want to get my Ph.D. and do work in research language delays and language development in children. the thing is, though, that psych degrees are versatile because it's such a diverse field...there are occupational therapists, industrial psychologists, cognitive neuroscientists, family and marriage counselors, school psychologists....and they all do very different sorts of jobs! i know i'm not being extremely specific, but that's part of the beauty of this field. if your interest is science, law, patient care, writing...there are great roles for someone with a psych major to fill (though many do require Ph.D.'s).
lastly, i don't think it hurts to change majors. is it on your transcript?? otherwise, med schools won't even see it.
good luck