That's a moment that could be played up in a story, too. Two characters meet, the first one mentions Dygan, the other says "Who?" and it's because the first guy is pronouncing the name differently. "Oh," the second guy says, "you're using the colonial pronunciation. He uses the imperial form." That way, we still don't know.
Haha, love it!
I say "DIE-gan". The shows' producers always seemed to give Cardassians very strong-sounding (to Anglophone American ears, at least) names, and I think "DEE-gan" would sound too... prissy? I think it sounds vaguely upper-class European (particularly French or German) and Cardassian names have never really struck me like that.
(Hopefully that made some sense...)