"New activation required"
Port out to Sprint MVNO with no activation fee and first month free (or money back guarantee), then back to Mint. Unreal mobile may be an option.
Porting to H2O (AT&T MVNO) may also be an option.
Current promo: first month free + free SIM (not sure about shipping). https://www.h2owirelessnow.com
Reference: H2O Wireless Offering New Customers Free Month Of Service, Discounted Phones
After a Mint plan expires, when does Mint cut off service if no plan is renewed?
Does Mint permit month-to-month service or must you renew to a multi-month plan?
Service is most likely suspended immediately upon plan expiration, but the account/phone number remains in an inactive state for up to 60 days before being canceled. You can still port out during this 60 day period.
https://community.mintmobile.com/other-21/what-happens-to-my-phone-number-after-my-prepaid-plan-ends-99
AFAICT, no month-to-month plans.
st3fx writes: "AFAICT, no month-to-month plans. "
I believe you're correct. It's a recent change, and month-to-month used to be an option. I recall reading that those who already were paying month-to-month could continue doing so, at least for the time being.
https://www.prepaidphonenews.com/2018/12/mint-mobile-is-discontinuing-some-plans.html