Enharmonic Spelling Key Signature

I'm having trouble trying to make a key change to the enharmonic equivalent of the new key signature when using a transposing instrument like Eb alto sax. I want to modulate from C major on alto sax (concert Eb major) up to Db major (concert E major). But whenever I make the transposition, the software only gives me the key signature of C# major and I'm unable to change it to its enharmonic equivalent Db major. Is this not possible?