How do I change the Security Properties from Signing: Not Allowed to Signing: Allowed?
Not possible with just the free Reader.
This can only be done with Acrobat Pro (but not Standard), or LiveCycle Reader Extensions.
I used to be able to do this before I installed the upgrade. Why would my security settings have changed?
If you installed Reader and uninstalled Acrobat then you need to re-install Acrobat. You may have both on your computer and when you updated reader it made Reader the default program for opening pdf files.