okay, well here's one for the electret mic. Everything to the left of C2 (including C2) is considered the audio amplifier circuit.
As for the otehr circuits, go here
http://discovercircuits.com/list.htm and look under audio, capacitor mic, dynamic mic, microphone, etc. u just gotta look around on the internet. google is also indespensable for finding electronic circuits n wut not.
i dont have a clue wut he meant by converting one from the other. you mite as well build a whole nother circuit if you're going to switch out mics. the circuit for an electret mic would be totally different than that of a capacitor mic.