I appreciate you performing the instructions provided above, @jahve.

Being unable to change a project name on a paid invoice can also be caused by inactive accounts on the COA which are associate in the transaction. To isolate this, I suggest checking your Chart of Accounts and activate the accounts that are used on the paid invoices. 

Here's how an account can be reactivated with just a few clicks in QuickBooks Online:

1. Click Accounting on the left navigation menu and choose Chart of Accounts.
2. Select the small Gear icon above the Action column and choose Include inactive.
3. Click Make active next to the inactive account. 

I recommend the following article for additional insight into this process, such as what happens when an account is made inactive: How to Delete an Account or Restore a Deleted Account.

If you haven't found any inactive accounts, I recommend contacting our Customer Care Team. They have the necessary tools like the remote-viewing session that can help determine the root cause of this issue, and generate an investigation ticket if necessary.

To reach them:

1. Click on Help at the top menu bar.
2. Hit on the Contact Us button.
3. Enter a brief description of the issue in the What can we help you with? box.
4. Press on Let's talk.
5. Select on Get a callback.
6. Key in your contact details, then tap on Confirm my call.

I'm also adding here some resources that you can check out about managing sales transactions in QBO for future reference: 

• How to edit a sales transaction form
• View sales transactions

I got you covered if there's anything else you need. Just reach out to me, and I'll make sure you're taken care of. Have a lovely day!