InfoPath Dev

We have a form that requries full trust that has been signed with a certificate and published to a SharePoint Form library.  Form has worked beautifully for all my users until today.  Today, a user tried to open an exisitng instance of the form and recieved the message "The signature is corrupt or the certificate used to sign this ...

I've been happily using the contact selector in a SharePoint hosted InfoPath form.  That is until several of my users had their "Office" designators updated in Active Directory.  My SharePoint site correctly grabbed the new information from AD, but the form where I have the contact selector is still using the old office ...

Samir,   Thank you for your quick response.  I am most definitly in Design mode, but the option is still missing.  

So a lot of the InfoPath how-to articles I'm finding out there require that I use the "Save as Source Files" option under the file menu.  However, I do not have this option (InfoPath 2007).  What am I missing?