Say for example, to upgrade to Walter's fixed SBF of 4.5.3_109 Kernel from 4.5.1-134_DFP-13XX, do the following.
1. Flash Defy-BL6Downgrade-CDT.sbf through RSD Lite.
2. Flash walter's 4.5.3_109 kernel sbf right away after that or an update zip package if you have a functional recovery. (Actually, you may flash any kernel. Even Froyo kernels)
Wipe data/cache in case you get stuck at boot. No more bootloader errors.
This is based on http://forum.xda-developers.com/show....php?t=1486731
Credits go to dlhxr.
Note: If you have an SBF which you want to flash after this, just the BL6Downgrade-CDT.sbf is enough.
If you want to get into a custom recovery (only if you already have it), then you have to flash BL6Downgrade.SBF, or you will end up with a Bootloader ERR screen.
BL6Downgrade.SBF [34MB] - http://www.mediafire.com/download.php?vdpp0nnr5f12e95