It is not possible due to the minimum and maximum code supported for those switches. The x480 can only be upgraded to 16.2 and the minimum version allowed on the x620 is 21.1.
Bad luck here. You need the same firmware on both units. But X620 starts on 21.x and X480 stays at 16.x. You can still use MLAG and EAPS, but each device will have own IP address and config. Z.