Ok, would you allow me to stick my nose in this? some historical linguistics shouldn't hurt too much.
Coming from Uruguay, in my young years (counting in dinosaur time) for 'y' and/or 'll' we used the sound 'zh' (I hope everybody knows the sound spelled in such a strange way). (Question: How do I insert the proper phonetic symbol here? I can't find it).
In Buenos Aires, they had the same 'y/ll' sound as Montevideo did. Slowly, in the 60s. it started to change (the change started actually in Buenos Aires, and then went South and East, meaning it reached Montevideo about 10 years later). The traditional zh became a [sh], as more and more people started to switch. I mean, young people in particular (which is normal for sound change).
Up to 20 years ago, in Montevideo the sound was still 'zh', but later on it switched so fast, I couldn't believe it (I go there about once a year, and visit Buenos Aires too. With my tape recorder, of course).
Everybody I know, now says 'sh'. Even my childhood friends, all grandparents at this point, are starting to change the voiced to the voiceless. Young people don't even know it used to be zh.
By now, all Buenos Aires and Montevideo are 'sheístas'. Considering there was never a sh in the language, that change doesn't confuse anybody.
About the other strong [dzh] sound mentioned, the one that appears in absolute phrase initial position:
- Quién está ahí?
- dzho
- qué tenés en esa bolsa
- dzherba para el mate.
also changes but not so drastically, and there is a reason for this:
If the dzh strong voiced occlusive becomes voiceless, it would basically become our normal 'ch'. That means the 'new' ch sound would be intruding in a real old phoneme of the Sp. language, and therefore there is more resistance to such a change.
Still, many studies on Buenos Aires speech claim (Harris et all) that the initial actually became a 'ch', and nobody used the dzh anylonger. I know that's too far, and I taped and analyzed those sounds, and yes, many people still hang on to the voiced consonant. Will it change completely for the next generation? maybe, maybe not. One thing we cannot know, is the future. It may vary again, go back, neutralize, assimilate, whatever you want. We cannot know.
saludos