If a package is loaded, the option setting code in the package is already processed and gone. Thus in general you cannot change package options later.
But you can specify options earlier, before loading the package. Apart from a global option (with side effects for other packages)
\PassOptionsToPackage can be even used before
Package interface for loading options
Sometimes a package or class allows the setting of a subset of options later. For example, package
Also some options of package
graphicx (or rather for
\includegraphics) can be set via:
However, the option
demo is not supported this way.