ND,
I didn't answer your secondary question as I do not have recent information but as no one else has volunteered an answer I will post my observations and let the experts shoot me down.
In the days of Air Eng, ALM, AEOp there was no interchange between them in flying specialisations although they may have been assigned some generic NCA ground jobs.
Now they are all badged the same I don't think it has changed anything under the surface.
To take your example, Acoustics to Crewman, while it would be theoretically possible I am not aware of it happening except in one set of circumstances and that was SAR.
SAR winchman is a specialist skill and not all crewmen can pass the training. When the SK entered service the SAR crewman role was opened up to Nimrod AEOp. The Nimrod AEOp was of course already well acquainted with maritime SAR so this was a useful skill to bring the the SK with its greatly enhanced SAR capability.
As I say, my information is well out of date.