If you need to change the edition of Server 2016 you can use the following command.
DISM /online /Set-Edition:<edition> /ProductKey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /AcceptEula
To check what editions you can change to use:
DISM /online /Get-TargetEditions
Here are the standard product keys for each edition.
| Windows Server 2016 Datacenter | CB7KF-BWN84-R7R2Y-793K2-8XDDG |
| Windows Server 2016 Standard | WC2BQ-8NRM3-FDDYY-2BFGV-KHKQY |
| Windows Server 2016 Essentials | JCKRF-N37P4-C2D82-9YXRT-4M63B |
| Windows Server 2019 Standard | N69G4-B89J2-4G8F4-WWYCC-J464C |
| Windows Server 2019 Datacenter | WMDGN-G9PQG-XVVXX-R3X43-63DFG |