If your old expansion board is of version 1.1, it should work well with the current version of OpenSprinkler. To make sure, you should check if the expansion board cable is polarized (i.e. has a bump on the connector). If so, then there is no problem.
Regarding the question about DIY kit: other than soldering and assembling the DIY kit, I don’t think there are other complications using the microcontroller-based OpenSprinkler, especially if you are using wired Ethernet.