I've thought such DSU system for a while, and it came to me that using a simple fork/exec technique require you to save (or dump) you data somewhere, before the exec() call. If you do it with a file and your data is huge, your update is no longer "seamless". :-/

Some research (a while ago) showed me that exec() on Linux call may be parameterized in order to prevent it to cover data segment (while I am not 100% sure), but then I think it exist only for Linux and is not portable.