Actually, it isn't about replacing the word as--it's revising the sentence in such a way that the word isn't necessary. That can take quite a bit of effort, and it is harder than it looks because you have to really think about it. I've been experimenting a bit with E-Prime (eliminating to-be words)--just eliminating one word (was), and only in the narrative. Since the word can't be changed to another word, I'm forced to look at the sentence and either revise it--sometimes quite drastically--or delete it.
If you want Word to flag 'as' for you--though it may drive you crazy--just set the autocorrect to capitalize it. If you do a search and replace, you must put a space before and after or you will have "as" in the middle of other words (i.e, mASh) highlighted.