I need to modify one procedure from within a package. I need to touch both the declaration and the implementation. As I am maintaining patch files for each modification, I would like the changes to be minimal.
Can I update the package with just the changed procedure ( if yes, how ? ) , or do I need to supply the complete package definition and implementation?
You need to replace the whole package specification and body - you cannot operate on just part of a package.
You cannot. As far as I remember, the only way to avoid referencing objects invalidation is not to touch your package declaration, and perform only CREATE OR REPLACE PACKAGE BODY.
Since the declaration changes, you might consider putting the new procedure into a new package, to avoid touching the existing one. Packages using the new version of the procedure must be adapted anyways, to reflect the change in the declaration (unless it's a new parameter with a default value).
If you're on Oracle 11g, and you want to minimise invalidations of other objects, make sure to place the new declarations at the end of the package spec.
Just to contradict everyone else . . .
Technically you could do it - you could write something that would take in your patch file, retrieve the existing package source from the database (using USER_SOURCE), apply your patch, and then recompile the package using EXECUTE IMMEDIATE.
However, I don't think it would be a very good idea - patch based fixing becomes very difficult to keep track of, especially once multiple patches and multiple databases are involved. Putting the whole file into source control is a lot better - your patch should still be clearly visible.
If the patch is to a third-party package, consider wrapping it - so that everything is a straight call through except your patch. Or put your patch into a standalone package that calls the first one. There is still a danger that a change to the original package could be incompatible with your patch.