What do you mean by Coordinate Signs? are you saying that Easting is provided as -1000.000 in stead of 1000.000.
I just tried scaling the project by -1 and that doesn't work so that doesn't help in this scenario
I guess Mirror Objects would do what you need - that is a command in 4.12 that you can run by typing mirrorobjects at the command line (F12) and that will let you Flip easily around the X or Y axes.
Mirror Objects will be a released command in v5.0 that was just announced last week and will be available sometime after Dimensions. It was a hidden command in 4.XX.
You can also do a Transform by Points - that also works - I did a test and created two points (-1000, -1000, -100) (-2000, -2000, -100) and then drew a rectangle between point 1 and 2.
I then created Point 10 and 11 at (1000, 1000, 100) and (2000, 2000, 100) (where I want the data to be) - this is a rotation of 180 degrees plus an X and Y shift into the correct location. It will work for Easting and Northing but not for height as that would just be a shift fro -100 to +100 but the data would still be upside down. So once you have rotated the drawing you can then use Scale and only scale the Z by -1 and that will change your elevations to the opposite sign (use baseline elevation of 0 in this process).
I would love to know why you are getting files of DXF or LandXML where the signs are wrong - can you give us any background here as to why you are getting that (I could see that if you did a Takeoff from a Skewed sheet that maybe the data would be in the wrong place and that could be why you are seeing that - let me know)
Alan