The short answer is: it depends.
You're probably ok with the existing speakers, but you might not be - you really want to have a chat to the maintenance techs who look after your avionics to find out exactly what's in there.
Additionally, the high/low impedance categorisation masks the fact that the microphone technologies are actually very different, and there are several different ones around. The reason the high-headset-to-low-aircraft needs a battery is that the standard high-Z electrets need to be line powered, but the (low-Z) carbon mics don't.
Most of the major headset manufacturers do a low-Z version, which they'll sell you on demand. However, not all of them offer a field-switchable solution: Lightspeed, for example, can only be changed over by the factory (in the USA, no less), and they won't send you the bits. If the converter box works for you, it might be worth seeing whether you can find an aux power output to plug it into: some aircraft have lighter (or similar) plugs supplying 12V; add a monolithic 9V reg and you don't need the battery.