The flip-flopping of these two particular pronunciations is actually fairly common in numerous of the various segments of the US population. Most of the people who do it can do it either way, but may use the "hard d" when they want to affect a "colloquial" manner.

People from some ethnic backgrounds seem to find that the "alveolar fricative" form takes a little more effort than the "stop" form, but sufficient numbers of those who shouldn't have a problem also do it when they're just feeling "a little lazy," so maybe one comes easier than the other for just about everyone (?).

HB may have used the switch depending on whether - or how much - he wanted to emphasize the "dialectic" nature/origin of particular songs, pretty much as some "country" singers do the same when they want to "sound redneck." If a particular "sound" belongs to the origins of a particular song, that's the way to do it if you can do it well, and Harry could. If it doesn't come fluently to you, it might be better to "modernize" an ethnic/vernacular song, with appropriate respect for its origins, or just not do it.

John