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"When" may not really be the right question. The pronunciation of |d| as |ð| or |þ| and vice versa is a common occurrence in dialect variation. In local vernacular speech it may have always been pronounced that way in some regions, no matter how it was written in formal "Classical" texts. Don't forget that even in ancient times there were more than a couple of variations of Greek (Attic, Doric, Ionian, Cretan etc.), with their own variations in pronunciation, spelling and vocabulary. Texts tend to be standardized fairly rapidly in copying, so even reflected in regional spelling conventions (which they may not be), ancient dialect pronunciations can be difficult to determine even if they're reflected in modern usage.