XC40 from base interior to memory seats

[IMAY2023]

Front seat module will make your car banned if it's connected to VIDA! So transfer the modules between the seats or move over the covers

This is not true at all.  VIDA "VOLVO" does not block the car if an ECU is replaced. VIDA just forces ECU reload for that replaced ECU under HW changes before allowing total upgrade.  
If the replacement seat module came from XC40,  it can be programmed via VIDA or just disconnect the power that goes into the seat controller before connecting the car to VIDA. 

My current interior is the base interior: only lumbar adjustment with the small knob, otherwise fully manual seats without heating, ventilation or other fancy gimmicks.
The interior I'm looking at has it all: heating, memory function, perhaps ventilated, electrically rear foldable headrests, etc.
Would they be compatible although my seats don't have all of the fancy equipment?
Or is it the same hardware in between seats, just with deactivated options?

First things first, 
Since your original seats have electrical lumber support, most likely it has a seat controller but it does not have the other motors and the wiring for adjustments. 
Look under seat you should see a rectangular ECU with bunch of cables connected to it.  
So, you can install the new seats and plug it in.  Seat movements might even work without changing the config in the CEM.  


Memory function:
There are position sensors built in each motor and reports the position to Seat controller. 
Memory buttons are connected to the Door module ECU.  
When the position is stored in m1 or m2, the door ECU send a message to seat ECU via CAN bus. 
You will need to replace your door trim part with the memory buttons, do some rewiring on the door panel harness or replace it with the one has memory option. 

Heated and Ventilated:
There is a LIN slave module under the seat that controls the heating and ventilation. That LIN slave is connected to CCM "Climate control" 
You will need to rewire one positive and one LIN from each seat. 

Rear foldable headrests:
The is a solenoid inside the rear seats that opens a lock that holds the headrest it up position then gravity does the rest. 
It requires two wires on each side. 1 ground and signal to the CEM.  The wiring is simple but you should check your CEM if it has the PINs.  There are different levels of CEM installed on cars depending on the equipment level.  Even if your CEM does not have that PIN, you can wire a positive with a press switch in the middle to fold them.