That is, are any of the operational levels designed in a way which writes over or otherwise destroys existing data on the disk? I looked around but couldn't find an answer to this question, and can't deduce it from the jargon used by Spinrite documentation.
The short answer is no. In fact, it will make previously unreadable blocks of data readable again.
Now for a synopsis of the long answer.. What Spinrite does is to test the operation of the disk at the hardware level. It does so by verifying the disk's ability to reliably read and write data. Because it does so outside of your operating system, using instructions directly at the hardware, it goes far beyond what Windows' built-in CheckDisk does. It reads and writes back to the whole surface of the disk. It reads each block, inverts the binary data and writes it back to verify writability, then re-inverts the data and writes it to disk, verifying again that the data is readable. This has the effect of not only verifying the ability of the disk to function properly, but also to refresh the data stored on the disk.
It's a fantastic program. I use it on all of my systems periodically, and on all customer machines when they first come into my shop. This has corrected many problems for me, including non-booting systems and periodic BSOD symptoms. If you're considering buying it, go ahead. Steve is working on updates to the program that will vastly improve the speed with which Spinrite works, and the updated program will be available for free to all owners of the current version (Spinrite 6.0).