I currently only use my Monzo card for getting cash from ATMs when I am abroad so do not have much money on the account. Do you need to have a minimum balance with the new upgrade which also includes a bank account?
I do not believe there is a minimum balance requirement, and I have not seen mention of one being introduced.
I can recommend trying it as a day to day spending card, it is really interesting to look back on the month and see where your money has gone. Even more interesting a few months later when you will have forgotten!
Nope, there are no requirements for keeping a certain amount on the account, the upgrade is just taking you from a prepaid account to a Current Account.