On RB333 downgrade from 3.10 to 2.9.51

Is there some way, how to downgrade from 3.10 to 2.9.51 on RB333?
If I copy 2.9.51 .npk packages through FTP to router and choose option Downgrade, it does not work. After reboot 3.10 is still there, most of 2.9.51 packages are gone.
Thanks
Ivan

v2.9 doesn’t support RB333

Just curious, why do you need to downgrade ?
What is the problem with 3.10 version ?