about mif file question

That's correct, make sure you delete those files with the Quartus project closed though. You can delete both /db and /incremental_db. By doing this you'll force Quartus to recompile everything.

If you only need to edit one value you don't need an on-chip memory to do this. If you instantiate the megawizard "lpm_constant" you can use the memory editor to modify it on the fly as well and not waste a memory block in the process. I do this from time to time to inject bits into my hardware.