我曾经是一个程序员。(但是现在不是了) I used to be a programmer. (But now I am not.) 我已经是一个程序员了。 I am already a programmer. 我曾经住在成都。(但现在不在成都了) I once lived in Chengdu. (But now I am not in Chengdu) 我已经住在成都了。 I have lived in Chengdu.

曾经 is used for some past state, normally we use this word in the Simple past phrase. for example, I was a teacher, but now I am a business man. 我 曾经 是个老师，但现在我是一个商人。 已经 means exactly already, so it is used for The present perfect tense. for example, I have already finished my job. 我 已经 完成了我的工作。

use 曾经 when state something already exist from the viewpoint of the process itself. use 已经 when state something already exist from the viewpoint of the result of the process.

曾经=once, did something in the past 已经=already, have done something e.g. 我曾经是个老师，现在改行当演员了。 我赶到车站的时候，车已经开走了。