ATTENTION! I'm not the author of this modification. All I did was just correct the script a bit to work on the new version of BLT.

Link to the original post - https://modworkshop.net/mydownloads.php?action=view_down&did=19567#description

Description from the original article (slightly changed):
"Was kinda boring on Overkill tbh. This lets you change the base difficulty. 
Minimum is Overkill, goes up to Death Sentence.
Wrote this really quick so its very likely to have bugs. You've been warned. Not my fault if you crash or the game freaks out.
Known Issues: Game restart is needed for settings to change. Not sure why."