SharePoint will not recognize the changed sAMAccountName until the following is executed:
$user = Get-SPUser -Identity "domain\testuser1" -Web https://siteCollectionUrl
Move-SPUser -Identity $user -NewAlias "domain\testuser2" -IgnoreSid
Correct the email address in Active Directory to match the desired email address. The UPSA will sync the value from Active Directory and push it to the Site Collection's User Information List which will overwrite any edited value in the UIL.