In the March release, we performed a security upgrade for user passwords. When a user logs in for the first time following the release, an upgrade to the encryption of their password is performed. The text from the release notes follows:
Starting with the March release, the next time each user logs in, a security upgrade will be performed on the storage of their password. This will be completely transparent to the user. When platform managers view a user's login history, an additional action called Security Upgrade will appear that indicates when the security update occurred.