You will still keep your plugin when you upgrade. The sessions just allow you to launch multiple clients, 1 client for each session key. Launching the same plugin on multiple accounts depends on how the plugin is operating internally when launched. Plugins by default can be used across multiple accounts yes, but sometimes developers will add code into a plugin to allow only one session use at a time. It sounds like the universal fight plugin allows you to use it with multiple sessions based on the last feature they list: "Endless lifelong connections". I'm not 100% sure about that, I would contact the developer support for that plugin to be certain.
