We're moving our design from nRF51822 to nRF51422.
What's the difference between these IC from the point of view of PCB layout and antenna design? Are they interchangeable from hardware point of view?
The nRF51822 and nRF51422 are identical, except for one difference:
otherwise, it is the same chip. This means you can replace nRF51822 with nRF51422 on your board and it will work equally well with your current software. As long as you have the same variant code and package code on both chips, it should be pin and software compatible. See the nRF51 PS 3.1 chapter 10 for description of package and variant codes.
For what is compatible with nRF51822 and nRF51422, look at the nRF51 compatibility matrix.
For those who want to develop BLE only with nRF51822, buy the nRF51 DK or nRF51 Dongle development kits. They both contain nRF51422, which you can use both for BLE and ANT. Then when going into production, buy nRF51822, which is slightly cheaper than nRF51422.