The Rock2 series is made of the SoM and the base board. The reason why we make it SoM plus baseboard is answered in the Rock2 SoM compatibility thread at radxa talk.
Rock2 SoM (left) and the Rock2 Square baseboard(right).
The initial idea for us to make the SoM is that there are many requests from the customs that they want something similar to rock pro/lite but with some modification or extra/less components. We make the SoM because the baseboard is easier to modify and less time to make the PCB and verify the system. Also, SoM is actually easier to manufacture because it has less ports, thus will save cost for the customers. Your question is actually the seamless upgrade for the SoM. We did not consider that (Actually we did but a standard interface will drop some features of the CPU - rk3288, so we choose a custom interface with all features of rk3288). So the answer is no. We can make sure that the case can be compatible but not the base board.
On the software side, the both the Rock2 square and full support android 4.4 and soon android 5.0. The factory firmware is android 4.4 since the Rock2 stable enough for daily usage. Andriod 5.0 is already running with full features on Rock2 series, we just need more optimization for the audio/video etc. For the Linux distributions on Rock2 series, we customize a Linux distribution called Rabian base on Debian 8(jessie). Why customize distribution? Because nowadays there is still no standard ARM distribution. We want to provide easier updates for all users. We don't want to flash a new image just for a new feature of the kernel. So we setup Radxa package repository and provide the packages instead of images for upgrading. Why Debian? Because Debian is stable, has longer life cycle and support time. It's better for us to polish our own distribution. Current status of Rabian is we have every ports working as well as Wi-Fi/BT, hardware decoding and 3d acceleration is not implemented yet.
The Rock2 SoM have the following spec:
The Rock2 Square base board has the following spec: