How can I have Word auto-replace words and abbreviations in a selection of
text I have cut-and-pasted from another source?
You can't. The AutoCorrect feature is triggered by typing a recognized item on
the keyboard. It doesn't scan pasted material at all.
The closest you can come is to (manually) go to the end of each word or
abbreviation that needs to be replaced, and type a space and then backspace. The
space will trigger the AutoCorrect, but then you need to delete the extra space.