In order to change stock things, you'll come somewhen to a point where you need to replace a existing texture or model but not a complete lvl file.
For this it is important how SWBF2 handles double files. This means you have two lvl files that contain the same texture/script/msh,... filename but those files themselves differ.

• *.tga - the last loaded version is used
• *.msh - the first loaded version is used
• *.light - the first loaded version is used
• *.fx - the last loaded version is used
• *.hud - double names are ignored, everything shows up (so if you copy the original 1player.hud to your custom ingame and load it, you have everything twice at the same position. You only notice at semi transparent parts that'll have half transparency now)
• *.lua - actually idk, but i think the laste loaded version is used as long as it is only read and not executed