Modifying Signed Documents, help with permissions

Question

Hello,
I'm setting acrobat, and i need help with the following.
For many reasons we want to be able to sign a document, but also make changes to it later on
but without the need of re-signing the document, i.e. when a drawing becomes obsolete we need to put a watermark on it.
How would i do this, as at the moment when we try to watermark a signed document, it says i don't have the correct permissions
Cheers
Stuart

Answer 1

I understand what you saying, but i know another big reason, that i know for a fact not just us would face, would be due to the following.
If we had a drawing from 2001, in which Bob was the author. Bob would have signed the document.
Now in 2009 and bob is not here, and we need to update the document, i.e. we need to add some holes, but the original part has stayed the same in respects to form, fit and function. A way we used to do this on the earlier acrobats, would be to use the replace pages feature which would keep the signatures and update the drawing with the relavant change details on it. Its not just as easy as putting a new name to the drawing, due to liability reasons. I.e. if the part was to fail, you need to know of the person who designed the part.
Is there not a master certificate or password that would give you these permissions?