Thanks to the other posters suggestions above, I found a solution!
In Word 2003:
Click the show/hides codes button (Image 1).
Below the letterhead place the cursor where you want to place the section break (Image 2).
On the menu bar, click Insert >> Break (Image 3).
The Break box pops up. Under "Section Break Types" select
Continuous (Image 4).
A ======== Section Break (Continuous) ========= line will show up (if you have the coding on) (Image 5).
SOLVED
Now you can adjust the margins below the Section Break Continuous line, paste text, etc., without changing the above letterhead.
Sorry to double post.