In reply to Legal to modify an existing retail font and sell it to a client?:
By and large, people are reasonable.
Last year I encountered a situation where a font was being used for headings and subheads in a project that was very special to me. I was really unhappy with the spacing (and kerning) of the font, and thought it could be fixed. I volunteered to do so, contacted the designer and copyright holder of the font, and they were quite amenable to me going off and doing my thing. If I recall correctly, I licensed a copy of the font to have it to work with, and delivered my modified font to both the user and back to the designer.
Now, it was *way* more work than made any sense, but I achieved what I wanted.