I have multiple images in seperate artboards in illustrator, all lines have stroke either 1pt (or no stroke, 0pt), but are different colors and styles. Is it possible to make the 1pt lines of weight 0.75pt throughout the whole document without changing other parameters?
You could try selecting one of the 1 pt lines and then using Select > Same > Stroke Weight. Then just modify the appearance to 0.75 pt.
Double check your selection when you do this! It's easy to accidentally modify undesired strokes this way.