Signatures and Design History
A digital signature does not include the contents of Design History. While a signature captures the current state of a model or file, Design History holds previous or alternate states. Thus a signature is not invalidated when changes are committed to Design History because commit does not modify the current state of the design.
You can use Design History to restore broken signatures that result from changes to a file, which causes signatures to become invalid. By restoring the original contents of the signed design, the signature again becomes valid.
Design History can also restore deleted signatures. If a signature is deleted, that is a change to the DGN file that can be committed. Design History can then be used to undo that change, restoring the signature.