I am not certain there are any free packages that accept AutoCAD, but I
the IntelliCAD vendors offer a 30 day trial that is fully functional.
I do recommend the IntelliCAD products. They are equivalent to AutoCAD
14, though they can import newer versions.
To be clear about IntelliCAD, the IntelliCAD consortium does not sell
the program to consumers. Rather, companies like BricsCAD and Cadopia
buy the license, add their own special stuff, and sell it to consumers.
Here is one other method that might work for you if the changes are
minimal. Get AutoCAD's free DWG viewer called True View (they finally
caved in after all the competitors were releasing DWG viewers). The
open and print the drawing to a TIFF file using the Microsoft Office
Imaging printer driver, or perhaps there is a raster printer driver in
the True View program.
Once you have it as a raster file, you can use another image editing
program to make minor changes.