There is no harm in changing Employee IDs as that as a unique identifier for each employee that you define. Most customers will match the Employee ID in Concur to the user's Employee ID in their HR system. Those changes can be made manually, through the Excel-based import, or via the 320 record type in the csv import.
The Report ID is another story. that is a Concur generated unique identifier and the format cannot be changed. Some customers find the 20 characters of this field unwieldy and hard to use. Many use the Report Key instead. This is also a unique identifier of an expense report, but the values tend to be easier to work with.
Are you using employeeID for Financial posting (e.g. to determine Employee vendor account)? If yes, then changing EmployeeID in Concur may result in failed postings in Finance if new master data is not available in backend.
What if previously "Vendors" are used - so Bills are created.
But then we are switching to Expense Reports - so this time we shall use "Employees".
Is there anything that needs to be changed? Like settings? Or preferences?
Employee ID is already updated in Concur to match the Employee's Internal ID in the system.
But when trying to enter an Expense Report, it throws an error "Invalid account reference key x"
x is the Employee ID.
We have employee IDs match the employee ID in our HR system. For those who don't have an employee ID in our HR system, there is no issue with changing their employee IDs to something else. A lot is dependent on the employee ID such as reimbursement so we don't change the employee IDs ever. The report ID is a unique identifer of an expense report and I am unaware of a way to change this to be unique to your company.