In model space you draw in drawing units (not inch, feet, metres, miles). You just assume that 1 drawing unit = 1 inch (your preference for drawing units).
Now if someone else has drawn the drawing on the basis of 1 drawing unit = 1 foot, then you will have to rescale the whole drawing so that 1 drawing unit = 1 inch. In this case the scale factor will be 12.
If the drawing was metric, then the scale factor will be different.....
from mm to inch...scale factor is 0.0394
from m to inch...scale factor is 39.372
You can't change your drawing units from inches to feet......it's either one or the other. If you started out using inches....then you must continue to use inches.