@cake765 overall it won't mess with existing devices, because you have to have separate profiles for Android(Device Admin or legacy model) and Android Enterprise. So it won't affect current devices.
Using Enrollment restrictions you can limit group of users who can enroll Android Enterprise devices and i do recommend to do that for proper testing/policies creation prior to enabling that for everyone.
Just be aware about one more thing (at least i saw that during my tests). If you have both Android and Android Enterprise enabled for user, all new enrollments will go through Android Enterprise. User has no option to choose the mode.
Best Response confirmed by
cake765 (Occasional Contributor)