If I sent a college my high school transcript while applying as a high schooler, do I have to resend it when applying as a prospective transfer student?
Depends upon if they consider your HS grades and test scores in your transfer admission so you may need to send them again. Some colleges will only want your college transcripts. You can look up the school’s transfer requirements on their website.
Contact the college, and ask if they still have your HS transcript on file, and whether it can be merged with the transfer application. Sometimes this is possible.