Are the scripts written on OS 10.5 portable without changes on 10.6 or newer versions of mac OS?
Yes, they should be compatible until Apple makes sweeping changes to the UI. It is possible that some images might need to be updated; at one point Apple changed the default size of buttons in the interface, and if you had captured the entire button (as opposed to just the label on the button) then the images of those buttons needed to be recaptured. But for the past several releases of the OS, scripts written against the older version continued to work against the newer version.
Even when dramatic changes are made to the UI, you may need to recapture some images, but the scripts themselves should not have to be changed.
New OS’s haven’t been all that painful the last couple of times. The thing you’ll want to watch is how Apple changes font smoothing. That is a perennial pain in the rear end. Also watch for changes in background color/patterns in the OS.
As long as you are capturing the smallest possible image that you need to verify, you should be largely fine. But, if as Matt said, you’re capturing the whole button instead of just the label, you may have a few more images to capture
What we do for a new OS is create a new text platform, then make a copy of our suite of images (in this case, 10.5) and then only replacing those that bomb a script.