Transition from Annual plan to Monthly plan after expiration date

I’m currently on an annual contract plan that expires on June 8. Since annual plans are being phased out, I’d like to switch to the Basic InReach Enabled monthly plan after that date.
Is it possible to select this plan now so that my account transitions to it automatically on June 8? Or would the change take effect immediately and would I be charged the monthly fee right away, even though I still paid until June 8 for my annual contract plan? Or would I be reimbursed for the remaining fraction of the annual plan?