I've found that if Word changes a word on you, you can just go past the word and then come back to it changing what you need and then just click back to where you were. It will only change a word when you press Enter or the space bar, so if you change a word and don't press one of these, then it will stay how you want it.

I've often found that when putting in an abbreviation with a period at the end it will capitalize the next word you type thinking that it's a new sentence. Just by letting it do this and coming back to the word you can get rid of this problem.