The query of WEBUSER has been changed to a Resource Query node, if your 8.0.3 flow has the DME query to WEBUSER the script that converts them to 9.0 will automatically convert then to the Resource Query. Change the flow back to the DME query and convert them to 9.0
The email will now be populated on the LDAP. If you do not have a process in place to update it there then you may have to go back to your user field option (or from HR11?).
Otherwise Query5820_OPE_USER_FLD_05 + "@holidaycompanies.com" is correct.. if USR-FLD-05 is a numeric you may have to put "" in front to ensure it converts to string as such "" + Query5820_OPE_USER_FLD_05 + "@holidaycompanies.com"
I assume you have a DME query called Query5820 that is called before you are trying to assign the value to EMAIL. And EMAIL is defined in your start node. If you are creating the EMAIL variable at the time of assignment then you need to use: var EMAIL = .....