Since 1284 is a pin compatible with 644, it should be an easy replacement. The source code should also compile under 1284 profile without problem.
However, keep in mind that if you want to use the bootloader (to be able to upload a firmware from OpenSprinkler’s USB port), you will need to modify the bootloader, and this will be slightly more complicated because 1284 has 128KB memory space which means the memory address will exceed one page. The bootloader code should be able to handle multiple pages. On the other hand, if you don’t mind using an ISP programmer to flash firmware from the ISP pins, then no need to mess with the bootloader.
With the introduction of the new 2.2u kit I did some reading and it seems the guys in the Arduino world is also using the OptiBoot loader for the 1284 chip. I wonder if a future 2.2u kit might be offered as a ‘2.2u+’ using a 1284 chip?? Hint.. Hint..
I already have my budget set aside for one if it ever becomes a reality.
You are right — with hardware 2.2 it’s now easier to upgrade 644 to 1284, because Optiboot for 1284 already exists. At the moment we are not planning to upgrade to 1284 yet, partly because we still have existing stock of 644, and partly because the current firmware, which is 54KB in size, is not approaching the limit of the program memory size yet.