Why did my files get changed from AIFF or WAV to m4a?

i purchased the majority of my music from a site called Traxsource which provides music for DJs. the files they provide are sold either as mp3, .wav or .aiff. been collecting the music for at least 10 years and suddenly on october 26 i noticed every single one of the files in my apple music library had been changed to either AAC or apple music m4a. what could i have done that would have caused this? ive update my library multiple times and have never experienced the files changing from what i paid for. this is very frustrating and is making me want to have a melt down. why just WHYYY would this happen? someone please help me out here