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There seems to be a new version of Microsoft Teams that is being rolled out will the new version eventually become the default version of Teams.
Even though you may have opened and been using the new version of Teams it will still prompt you if you want to open the new Teams.
New Teams is a new program. In my experience, classic Teams is not auto-uninstalled and shortcuts to it are not removed from the bottom tray or the desktop, so when you open Teams from there, it opens classic Teams and prompts you to switch. You have to replace the shortcuts to open new Teams straight-away.