The parts are pin compatible, but since the 16F628 has more peripherals, the code written for 16F84 will not be able to access them.
The memory map is different, too. That means that code written for F628 may not work correctly on F84.
The safest way is to copy the .asm file, change the processor type and reassemble the code. That way, in case there are problems, you get errors/ warnings, which help you fix the subtle differences (but which can prevent proper operation).