I don’t think it would work — while 1284 is pin compatible with 644, the circuit on OS 2.1 has no built-in USB-serial chip — it assumes the bootloader on the chip is the USBasp bootloader. Unfortunately there is no USBasp bootloader for 1284, it’s only for 644. So even if you replace the chip, there is no available bootloader for it to work — you would still need to program it using ISP.