I had a fear of losing my account balance in Checkout 51 after I logged out of the account or when it crashed a few weeks ago. Even when Appstore wanted to upgrade my Checkout 51 app, I was afraid that I would lose my account balance. But after going through multiple Checkout 51 upgrades, uninstall and installing the app again, logging out and logging back in again, I can safely say that you will not lose your account balance if you do any of the aforementioned actions.
Your account balance is stored under your email address or Facebook account id in the Checkout 51 server. The balance is not stored on your apple device. So you can technically move around devices and log in under the same account name. But you cannot have more than one account on a device.
Visit Checkout51.com for more terms and conditions. You can download Checkout 51 for free from Checkout51.com or from the Apple Appstore.