Users of classic Teams will be moved to New Teams soon.

Microsoft Teams has announced that it will automatically upgrade classic Teams users to new Teams after March 31, 2024. Though the upgrade will not apply to Virtualized Desktop Infrastructure (VDI) users, be aware that classic Teams on VDI support will end on June 30th 2024 as well as for the regular desktop. 

If you are a classic Teams user, you will at first see banner messages encouraging you to move to the new client. If you postpone the upgrade, you will have to do the upgrade on June 30th 2024 

If you are an admin and want to control the process, take a look at the following article on how to use policies in Teams Admin center.