Inhalaattori
Emperor
Quick response:
https://twitter.com/FXS_MisterKevin/status/1306679954059522050?s=19
Units continue to be resource-maintenance free after upgrading. They will incur gold-maintenance after upgrading though.
They are not actually maintenance free to begin with. You can see from the first look video that they cost gold maintenance just like regular knights. This is also clear from the leader screen text. There is nothing about them being maintenance free. They dont require resources, but they do have normal maintenance costs.