The specification says:
A byte range digest shall be computed over a range of bytes in the file, that shall be indicated by the ByteRange entry in the signature dictionary. This range should be the entire file, including the signature dictionary but excluding the signature value itself (the Contents entry). Other ranges may be used but since they do not check for all changes to the document, their use is not recommended.
This seems to allow that you first create a signature for the original PDF and then append a new revision holding that signature indicating that range of signed bytes only contains that original revision, not the extended revision without only the signature.
In reality, though, PDF viewers (especially Adobe Reader) will only accept signatures which follow the recommendation that the signed range should be the entire file, including the signature dictionary but excluding the signature value itself.
Newer specifications, e.g. the ETSI PAdES specification ETSI TS 102 778 (cf. section 5.1 item b in part 2 and section 4.2 item c in part 3) even make this recommendation officially a requirements, and so will ISO 32000-2.