Open invoices not showing up under customers after upgrade

Welcome to the Community. I'll help you sort this out, @FairFun.  

You can create back up from the old version and restore it to the new version. Let me walk you through:  

 Back up your company file manually:  

1. Go to the File menu and choose Switch to Single-user Mode. 
2. Click the File menu again and select Back up Company. Then choose Create Local Backup. 
3. Choose the Local Backup, and click Option then select Browse where you want to save your backup file. 
4. Set the number of backups you want to keep. This is optional. 
5. Click OK when ready to save. 
6. Tap Next, then Save it now and Finish.  

A pop up message will appear to verify the data is already successfully back up. Then Click OK once done. 

After backup company file you can now use to restored your company file. To restore the file has a .qbb extension at the file name.  

In case, error persist when restoring backup file, here's how to fix them. 

Please refer to these articles for more detailed on how to Backup and Restore your company file: 

• Back up your QuickBooks Desktop company file 

• Restore a backup of your company file 

That's it. This should help you point in the right direction. Let me know if you have further questions. I'll be here to answer them. Have a great day!