If you're in grace, your Google Play Reward balance will remain in your wallet but you won't be able to use it. To use it, you need to reactivate your ookyo plan by manually paying for the renewal. Via the ookyo app > HQ > go to Maxis Pay to renew.
If you terminate your service or port-out, your Google Play Reward will be forfeited. You won't be able to use it any more and you're not able to transfer it out.