Notify users of upgrade rather than pop up a modal dialog
Rather than pop up a dialog that will launch an upgrade on default click, display an available upgrade notification as a horizontal bar on the main login page of the UI with an embedded button to launch.
A dialog makes power users like me launch an upgrade at the moment we need to access a password (I start typing before I the app is done loading as I know it well), and is really annoying. Making the upgrade a more deliberate choice avoids the window silently disappearing when not expected and makes upgrade scheduling easier.
It makes the default app behaviour flow different when an upgrade is waiting in the wings, which breaks basic UI function assumptions and makes the experience inconsistent.