Just to double check where are you editing the email addresses for you customers, are the sales items e.g. invoices, sales receipts or statements still gong to the same email after generating new items?
Hope to hear from you soon,
Testing on my own QuickBooks Plus file, I can see by changing the customer email this will apply to only new invoices. Please check for me that you are not expecting the email to change on old ones being resent.
If it is not changing on new ones, please reach out to your friendly support staff by clicking here.