After the change, there will be no sign that you see the original document.The result of the signature validation is displayed in the status bar and within the Digital Signature dialog.When you sign a document with Open 3.2 or Star Office 9.2 or a later version, and you open that document in an older version of the software, the signature will be displayed as "invalid".Signatures created with older versions of the software will be marked with "only parts of the document are signed" when loaded in the newer software.When you sign a PDF document, then this marking is not used.Signing only parts of the document is simply an invalid signature.Signing other document formats is not supported at the moment.When you load an ODF document, you might see an icon in the status bar and the status field in the dialog that indicates that the document is only partially signed.
When you apply a digital signature to a document, a kind of checksum is computed from the document's content plus your personal key.
The checksum and your public key are stored together with the document.
When someone later opens the document on any computer with a recent version of Libre Office, the program will compute the checksum again and compare it with the stored checksum.
When you sign an OOXML document, then the signature will be always marked with "only parts of the document are signed".
Metadata of OOXML files are never signed, to be compatible with Microsoft Office.