HippyK Thank you for your detailed explanation. If I understood it correctly, you have to change the WIFI each time you change floors. If this is correct, I assume that your boosters do not have the same network name and therefore you have to change the network each time.
Basically your WIFI device does this itself. If the coverage on WIFI 1 on the upper floor gets worse because you go downstairs, then your device should switch to the stronger WIFI. This will also result in an interruption since you are not using a WIFI mesh system.
If all my statements are correct, you can use WIFI cloning as already written above. This takes the WIFI from the modem and copies it to the boosters with the same name (SSID). Then you have one WIFI name for the whole apartment. You can go up and down and your device decides itself which WIFI it takes.
If you do not want an interruption and build a complete strong WIFI network, then there are paid WIFI Mesh systems at the retailer, which improves your WIFI and this without interruption when changing one access point to another.
Greetings
Daniele