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Changing sub customers' email addresses in the Project is just a few clicks. I'm happy to guide you with the whole process. 

Here's how: 

1. Go to the Sales tab on the left side. 
2. Select the Customers button. 
3. Find and click the sub-customers that have a project. 
4. Hit the Customer Details menu, then tap Edit on the right side.
5. Modify the email address, then choose Save. 

Please note that whichever customer on this project, that customer will receive the invoices. 
 

If the issue persists, I'd suggest doing primary troubleshooting steps. Large data in the cache results in QuickBooks working unexpectedly. To start, let's open your QuickBooks Online Account in a private browser. These browsers won't save any cache or histories. 

Use these shortcut keys: 

• Press Ctrl + Shift + N (Google Chrome)
• Ctrl + Shift + P (Firefox)
• Control + Option + P (Safari)

Once signed in, go back to the Customers page, then check if you're able to change the email address. If it works, return to your default browser and clear its cache and cookies. It will stimulate the system and eliminate older data, which induces viewing and performance concerns. 

However, if the same problem, I'd recommend using other supported browsers.

Please browse through these articles below. These links contain multiple steps on how to convert sub-customer into projects, track your hourly time cost, etc. 

• Track income, costs, and profitability by project
• Projects FAQ
• Manage your customer list

Let me know how you get on after the steps. I'll be right here if you need anything else. Have a nice day!