Ability to use User ID as the Upsert Key for User sync via Connectors 2.0

Currently, in Connectors 2.0 for our User Sync job the upsert key box for User Id is grayed out and we are forced to select another option. Our Salesforce team uses firstname.lastname for user names and emails are firstname.lastname as well….so anytime someone gets married, we get a duplicate user in Gainsight. We’d like the option to use the User ID as the upsert key, so any name changes would flow through seamlessly.