Your cash balance is found in your Billing Summary (see blue box in image below).
If you upgrade, downgrade, remove numbers etc., you might have a cash balance. This is because we now prorate accounts. To see past information, click on View History. The Cash Balance History table will appear. If you have a history, you will see the transactions.
If your discount is larger than your new billing, you would be given a credit on your billing which would be applied to your first payment of your new plan. If you still have a cash balance, you would be able to use it to buy another phone number or additional buckets of credits at that time.
For more information about billing, check out this helpdesk article.