I know of several bands with long, multi-decade careers, during which they released songs and albums in a wide variety of musical styles. But they were still, in some fundamental sense, the same band, so I hesitate to call them bands that “underwent big transformations.”
I’ll mention Renaissance. The group still was a mix of classical and rock (though the first album was more baroque classical, and changed away from that), but it had a complete turnover of personnel between the first album and the second.*
*There was an album between the two with the original lineup, but it wasn’t released until after the second.
Thanks for mentioning! I had completely forgotten about them. Was really into them in my college stoner days, which is probably why I had to do a You Tube search to refresh my memory 
The |Beach Boys went from 2 minute surf and hot rod songs in the early 60s to Pet Sounds & Good Vibrations in just a few years.
Pantera was originally a straight-up 80s glam metal band. Aside from the distinct guitar, would anyone even recognize Dimebag Darrell from this?
