Changing only some words must affect the rest

Question

I have a specific document in which I have to make only some changes. The rest of the document is same. So far I've been using text highlight color, in order not to forget to make any changes but it's a long document, my first change should affect the rest. I'll give an example:

I give the letters to Jack. ... ... ... Jack took the letters to the post office which is located at Oxford Avenue.

The bold parts should change whenever I make a change in one of them. I tried to use Developer Mode and Rich Text Control but I couldn't link the words that I want to change.