This happens to me a lot. Luckily I have kind of gotten used to it at this point.
I really liked the old look of the blog a lot when I did it, but yesterday I decided I wanted something "cleaner" (this is a kick I have been on in general lately, I think it is a reaction to the horrific state of my house at the moment). I found this layout and was sold when I saw how neat the sidebar and posting area are, compared to the old layout where everything kind of ran together.
So I clicked, and *poof* all my hard work from the first time around was gone. And it didn't even hurt, really.
I fixed up the header to match, and now I just need to tweak the images in the sidebar and the transformation will be complete.
If only I could fix my house with a click!