You may find this helpful: http://php.net/manual/en/language.namespaces.rules.php
I like to think of it like a file directory so changing namespace is like changing directory; you can always reference the current directory or an absolute path from the root.
I must admit, I find it easier to just use a fully qualified namespace to access anything outside of the current namespace. It's a bit of a pain that any use declarations only apply to the current file. For this reason, if you ever had to move a class to a different namespace, it could be a bit of a pain.
If you use the PSR-0 standard instantiating an object would look like:
$object = new \\Vendor\\Package\\Class($foo);