Content type "System Page Layout" at "/" is sealed.

Scenario:
I was experimenting with OOTB Document Content Type by attaching a new document template to it and got the following error

The content type "System Page Layout" at "/" is sealed.
The content type "System Master Page" at "/" is sealed.
The content type "Office Data Connection File" at "/" is read only.
The content type "Universal Data Connection File" at "/" is read only.
The content type "System Page" at "/" is sealed.
The content type "Unknown Document Type" at "/" is read only.
The content type "Page Layout" at "/_catalogs/masterpage" is sealed.

Reason:
One of option while updating the content types is If you want to Update all content types inheriting from this type? which is defaulted to Yes

Basically Document Content Type is a parent to a lot of other content types , and few of them are sealed and cannot be changed. So selecting the above option to Yes will not let you proceed and will cause you this error.

Solution:
Set the option to No