How to decrypt user passwords

We have a web application that uses .Net 3.5 and we are about to finish a new version based in .Net 4.0. Our system already has active users and we would like to get them from our old app and copy them to the new one. The web.config entry for the membership provider specifies that passwords are encrypted (passwordFormat="Encrypted") and I am sure it is possible to get its original value because our old system also let our user to recover their passwords. We will ver much appreciate any information about how to decrypt passwords from old app in order to transfer user ti our new app.
I am including a copy of the membership and machineKey tags from our web.config file for your consideration.
<machineKey validationKey="our validation key here" decryptionKey="our decryption key here" validation="SHA1" decryption="Auto" /> <membership defaultProvider="AspNetPgSqlMembershipProvide ...