Greetings,
Yes, you cannot simply change the nodata value of a raster dataset, either in SDE or on a file based raster dataset. The only way to change the nodata value is to rewrite the dataset and change the nodata value in the process (e.g. in copy raster).
Simply changing the nodata value, especially in SDE, might create issues, so be careful changing something through the DB rather than ArcObjects.
Robert