If you do a Save As of your original project and then re-edit it as needed, you'll be using the original media files again, so there will be absolutely no reduction in quality -- and you'll still have your original project.
However, yes, if you try to re-edit your finished DVD, you're a couple of generations removed from the original footage, so yes you will definitely have reduced quality in your next re-edit.
Thanks, Steve. When I open my original ('long') project, do I then click 'Save a Copy' or 'Save As'? I'm just anxious not to lose the original as I begin editing work to make the 'short' (45 min) copy.
The results will be the same either way, Roger. You'll create a duplicate of your project -- so you can edit one copy and save the original.
Just a supplemental comment....
In your File Menu/Save A Copy, it would be a good idea to change the name of the copy before the save and note its save location.
I suggest this to avoid the possibility of accidentally overwriting the original project.
Thanks, ATR. Helpful reply much appreciated
Many thanks, Steve. Helpful reply much appreciated