Upgrade to enterprise

I'm not sure how to understand your question correctly.

There's no "upgrade" and every Android device is possible to use (in general) as an enterpise device and you can buy any device you want. I recommend to buy devices which are Enterprise recommended as a company which can be found here: https://androidenterprisepartners.withgoogle.com/devices/

Here's a general guide for "starting with enterprise": https://support.google.com/work/android/answer/6396012?hl=en