I agree with Sebastian, having a greyscale export option would be great if you wish to print using only K.
In the meantime, one way of solving this problem outside yEd is to export the diagram as a SVG file and open it in Illustrator. Then select the entire diagram and choose Edit > Edit Colors > Convert to Grayscale. Then save the file in a suitable vector graphic format, like PDF or AI.
This is my experience when working with yEd 3.9.2 and Illustrator CS5 on Mac OS 10.6.8:
If exporting as EPS or PDF from yEd and keeping the font embedded and opening the file in Illustrator, the text is flipped! This is also seen when viewing the PDF in Acrobat.
This is not an issue when the text is being vectorized (which can be selected when exporting), on the other hand doing so the text cannot be selected or searched, and the shape of the characters might be altered.
When exporting as EMF, the text is correct, but the size of the art board needs to be changed in Illustrator, otherwise the diagram will be cropped.
When exporting as SVG however, the text is correct and so is the size of the art board.