Viruses do have the ability to isert foreign DNA into a host - that’s one way gene therapy works. However, to do something this radical, the DNA would have to be altered quite early on during development - the more radical the change, the earlier. Also, you’d probably be talking about simulatneously changing dozens or more genes in different areas of the genome, wich is more sophisticated than we can do now.
If you’re talking about rearranging stuff in a fully developed adult, that would be much more difficult. You’d have to target some cells to die, and target other cells to begin development in new directions. I’d say that would require much more information and direction than could fit in anything we could call a “virus”, by an stretch of the meaning.
Theoretically, perhaps. Again, that’s how gene therapy works. Engineered viruses deliver needed DNA segments to cells. There are plenty of known natural viruses that have no noticeable affect on their hosts. But I’m not aware of any natural viruses that are actually beneficial.