How do I change the income account to a sub-account associated with an existing product/service when the historical transactions box disappears for sub-accounts?

Hello, userlafletcher138. 

Once you changed the income account as a sub-account, it'll still show in the income column. Since you're unable to see the historical box, you can try using an incognito window to identify if it is a browser issue. 

Please follow these shortcut keys to disable the web cache:

• Google Chrome: press Ctrl + Shift + N 
• Mozilla Firefox: press Ctrl + Shift + P
• Safari: press Command + Shift + N 
• Microsoft Edge: press Ctrl + Shift + P    

If you're able to see to choose the sub-account, you can go back to your regular browser and clear the cache. It allows your browser to function more efficiently. You can also use a compatible browser with QBO. 

The following articles will help guide you more in managing your product and service item types in QuickBooks:    

• Change product and service item types in QuickBooks Online 
• How to change the type of an existing parent account or sub-account  

Get back to me if you have additional questions or concerns. I'll be more than happy to lend a hand. Have a great one.