I just tweeked a template with the designer pro and would like to apply it to my existing shopping cart. Can this be done without having to rebuild the cart's contents? Please show me the way!!!!

Yes. It works fine. I've gone between a couple of themes. I tweak one while my shop is up live under the other.

Is it as simple as opening the current cart and selecting the desired change to template and saving?

Pretty much.
Export your theme in designer then select it in creator.

Okay, I'm doing the happy dance

It was not quite as simple as I was thinking. For what ever reason, I ended up deleting my cart folder(on purpose) on the server and recreating. I also deleted all the files on my computer of the orginal template and resaved as the new template name. Published the new files to the site and bingo- cart with the tweeked template is proudly displayed!

Another tip that might help anyone else trying to do this:
I noticed on the tweeked template- I had to go back into the server settings and change the remote path to my cart folder. It defaulted to "shop" when I changed from the orginal template to the tweeked template.

Thanks for the replies Eric!