Here's how I handle this issue -

Move the email address from the Email field in the form for that Contact to the Notes field. This way you preserve the old email address and Gmail will not show it in the auto-population list

Here's how I handle this issue -

Move the email address from the Email field in the form for that Contact to the Notes field. This way you can preserve the old email address and Gmail will not show it in the auto-population list. The contact will also show up when you search.