Well, my best suggestion is to make a copy if the app and try changing the key field. Then if you are OK with the result, make the same change in your live app.
There may be another way without resorting to actual script, but it gets a bit convoluted.
You could set up a cross reference table of employee Record IDs to userid's. Set the key field to be the userid.
Then set up a saved table to table import to copy across the employee records to the cross ref table where the userid is not blank.
Then set up a relationship between the cross ref table and the task table based in the userid field in Tasks.
Lookup the record ID number if the employee.
Use that to be in a relationship to lookup employee information from the EE table.
Set up a Webhook to rerun the import table when any EE record is edited.
It's a pretty convoluted solution.