To convert into 20" wheels ETRTO 406 (there is another 20" size = ETRTO 451 that won't fit on the Birdy), you need to rebuild your wheels = keep the hubs and replace rims+spokes+ tires.

If you want to keep your original 18" ETRTO 355 wheels, you need to also buy hubs.
The rear hub is a standard 135mm wide quick release 5mm hub.
But the disc front hub is specific for the Birdy, you cannot use any standard 100mm wide hub because the disk won't be at the good position (the front disc of the Birdy is not only on the right side of the bike instead of the left side as usual but its distance from the center of the wheel is also different). There is a special Birdy disc from Hubsmith, there is also a special Birdy disc Shutter Precision dynamo hub.

As I already explained above, there is a limitation in tire width when using ETRTO 406 wheels = max width is 28mm meaning that there aren't so many tires available (Schwalbe Pro-one and One, some Continental tires, note that with the rims I tried, the Pro-one were wider than 28mm and too wide to fit on the Birdy with front mudguard). All these 28mm wide ETRTO tires are road high pressure tires.