You can put in a support ticket to have that changed for new users, BUT if you have anyone still using Classic they will also be switched to the New Experience. So you'll want to wait to submit that ticket until after you have all current users switched.
Pull the plug!!! It's totally worth it. We moved all out most active users over carefully and then called IT to have Classic disabled like Sarah and Heather are saying. Then I sent out a mass email/training info for the NWE to everyone who we didn't train.
They were totally fine.