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.