There is a toggle switch at the top of the page in either version to flip between the two. If you are in the new version it is here, click the three dots next to your company or school name, if you are in the classic it is permanently visible top left of your screen.
I get that this defeats the Microsoft objective somewhat, and flipping back means I won't have the major differences between the two versions, which are time saved in switching accounts / loading pages / starting the app / etc, plus less memory used on the machine. However, with the memory saving being 50% of a very small amount of memory being used to start with, and the time savings being a few seconds per operation, I will not be missing out on much in my opinion.
This page tells you all the differences, and there is an easy video around 25% of the way down the page that quantifies them.
https://techcommunity.microsoft.com/t5/microsoft-teams-blog/introducing-the-new-microsoft-teams-now-in-preview/ba-p/3774406#:~:text=With%20the%20new%20Teams%20app,up%20to%2050%25%20less%20memory.