i tried out your suggestion Greg. But for some reason, the OnAfterChange is triggered before the signed state of the form is altered.
I put in a
thisXDocument.UI.Alert(thisXDocument.IsSigned.ToString());
statement at the end of the eventhandler for OnAfterChange and the value in the messagebox is not the final value.
e.g. when signing the form, the box pops up with a 'false' vaue and when removing the signature, it pops up a 'true' value.