Question [Q] Downgrade bootloader 4.5

I want to install Custom Rom and i came to know that bootloader version should be 3.xx or below
My phone Details is below:

Model: HTC HD7
carrier T-Mobile US
Os version 7.10.7720.68
firmware version number: 2250.21.40503.531
Hardware revision number: 0002
radio software version: 5.69.09.29a_22.50.50.21U
radio hardware version: A.12.0.S4
Bootloader version: 4.5.2250.0(129966)
Chip SOC Version: 2.2.5.0
THANKS IN ADVANCE
And This is my first time too.