Cannot remove invalid signature from expired certificate

Please someone tell me there is a way out of this nightmare!!!

I want to reuse a long and complicated form I last used and signed 2 years ago.

I am the only signer. I just need to change a couple of dates to reuse the form.

The form is subsequently signed by a second person.

However, my certificate has since expired.

I get the error message that the form has an invalid signature.

I have tried trusting the expired certificate, but even after that there seems to be no way of removing my now invalid signature.

If I try to Validate All Signatures I get:

Here are the relevant screen shots.