1 Downgrade from 3.50 to 3.15 debug with the same method of downgrade (3.15 debug fw must be in root of usbflash with 1st self)
2 Downlolad retail 3.15
3 insert usbflash with retail 3.15 and 1st self
4 turn on and wait for shutdown
5 insert usbflash with 2nd self and wait for shutdown
6 Enjoy for 3.15 retail ))
7. You can update to 3.41 officially from xmb
Ps.
Tested on 250gb eu slim with preinstalled 3.50
Or You can downgrade from any 3.50 to 3.15 with working bluray movies