Will upgrade mess with customizations?

I started out with WP 2.9 and Constructor 0.8.2. I did a ton of customization to the theme, the css, the php files, etc. as a learning exercise. I like the changes so much that I’m keeping them!

I updated to WP 3.0.1, and now the top menubar is not in the correct spot, and the NexGen gallery pages are centered rather than lined up like they used to be.

Now I see there’s an update to Constructor to 1.0.6.

My Question: if I upgrade Constructor, will all my changes be lost?

I’m actually a noob when it comes to WP, so I’m afraid I may not be able to re-create what I did if an upgrade resets things to default.

Thanks for the help!