When you start connecting to your own solution to Quickbooks Online we highly recommend that you work with a backup of your solution first. We also recommend that you use a different demo-only QuickBooks Online account - a Sandbox account. By using a Sandbox during development and testing, you will not have to worry about errors screwing up the data in either your FileMaker solution and/or Quickbooks Online account.
Switching to Live
If you are using the Starter Solution ( ie LedgerLink.fmp12), then that's where most of the code changes will be. That makes it easy to just drop in your live copy when you ready to go. Even if you aren't using the Starter solution, placing all the sync code in a separate file is a good idea, for this and other reasons.
Demo QuickBooks Online Accounts
The simplest thing to do is to just get another account temporarily. You can usually get one for 30 days for free. You can have as many accounts as you like. They all be accessible to you through LedgerLink. You can even switch which accounts has a license.
If you already have a regular Account, you can sign up at https://developer.intuit.com/ and get a Sandbox Account. These are free, but they have data in them. LedgerLink does allow you to Connect to a Sandbox account, but you need to sign up with a regular account first.
More information on Sandbox accounts can be found here:
You will need to log in first.