Intermission Screen Backgrounds

Can anyone tell me how to change the background screen textures in
Doom, Doom II and Final Doom.

I have tried to change the backgrounds but to no avail.  I want to
know if it can be done thru use of a BEX File or are there other
methods avaliable for such a task.

I know the syntax is BGFLATE1 etc (Doom) and BGFLAT06 etc (Doom
II/Final Doom); can anyone help me.

TIA

Sir Jimmers