Here's what their documentation says:
Changing Your Subscription Now
If you’ve used up all of your talk, text or data allotment and want to change your subscription early, you can do that as well. If you’re changing your subscription immediately, here are some things to keep in mind:
- A new payment cycle starts when you activate your new subscription.
- Your subscription will not be prorated, which means that any days or benefits (data/voice/text) remaining in your current payment cycle will not be added to your new subscription
- Any earned Rewards will be applied on your next renewal after the subscription change.
- If your services are on hold you will not be able to change your subscription until you re-subscribe. You can learn how to re-subscribe here.
Looks like you won't unless you contact @CS_Agent for one.