Because of the auto migration, you’ll still get data feed into your Analytics account. That data may or may not be accurate and you’ll probably have to roll back the migration or start from scratch. You’ll certainly want to be using the new GA4 code on your sites  and apps as soon as possible.

It’s best to get in front of this if you can, as rolling back the migration will be more work than starting scratch and starting from scratch may mean you have holes in your data. Which approach you takes depends on what state your account is in.