Sylver - 4:13 pm on Jul 9, 2010 (gmt 0)
I think the idea behind sandboxing a site-wide change is to prevent the situation where someone sells a web domain to someone else who changes the website completely and therefore takes undue advantage of the existing links.
To you, the content might look the same, but as you went with css, I expect that the sequence of items is significantly different, and that could be enough for Google to decide that the content is too different to warrant the same rankings as before.
You can test that by grabbing one of your old page, stripping the code, then taking the equivalent new page, stripping the code, and comparing both with a difference engine.
Try the (excellent) Google difference engine based on Meyr's diff algorithm Google-diff-match ([code.google.com ] - click on the diff demo)
This should give you some insight on how different your new pages are, from Google's viewpoint.
I don't know if Google Search uses this same difference algorithm (who knows?), but it is considered to be a very efficient way to compute differences and I wouldn't be surprised if it did.
Google handles a crazy volume of data and they can't afford to use processing-intensive algorithms. Implementing an algorithm to recognize differences the same way a human does would be prohibitive in terms of computing time.
In a classic diff algorithm, difference can be measured by the number of characters that need to be changed in the old document to get the new document. Take a sample text and scramble the order of the paragraphs. You will see that the result will be considered to be completely different in Google-diff-match demo.
My point is that even though the content is "the same" from a human's viewpoint, it could easily be computed as being mostly different or even totally different depending on the way the code was rearranged.
Test several of your pages and see how badly different they are from what they were previously. If they are very different, try to rework the code sequence to make it more similar to what it was. CSS gives you a lot of freedom in regards to what goes where in your code, so you should be able to change things to match pretty closely. It is a lot of work, but it might be worth it.