Will the total port days at an M-port change after importing a cargo that is loaded/discharged at this port?

No, the original total port days at the M-port will be preserved.

The number of extra port days (XPD) at the port will be automatically adjusted by the system after the relevant cargoes are imported, so that the sum of port days (as imported via the CP terms of the cargoes) and XPDs, will still equal the original total port days at the M-port before the cargo import.