are you sure you modify the right copy of avr package? one is in IDE installation folder but if you installed an update the one with IDE is not used. the update is in the Arduino15 folder (you can get there by clicking on the preferences file link in IDE Preferences)
C:\Users\andrassy\AppData\Local\Arduino15\packages\arduino\hardware\avr\1.8.5
You do not get it... mucking with the IDE is not something you should do; it is like going into Windows and screwing with the registry; lot of bad things can happen.
When we answer questions like yours, other novices read the post and think it is perfectly OK to do the same, this simply create a future issue for forum support.
I see far too many inquiries like yours that drag the discussion off-road and into the ditch. No one is denying you the knowledge, but putting a "do-not-do-this" into a public forum is just an invitation for others to sail into dangerous waters.
Ray
we experiment. the final solution can use a separate board definition in the hardware folder
ARGH ... a co-conspirator 
@mrburnette I understand your concerns. However, it is not my choice to modify the IDE.
@Juraj I cannot thank you enough for all your help all these day!!! It finally worked! I had to edit the right files, which were in a different directory!
After changing the boards.txt definition for pro mini:
Then in platforms.txt, this is the command that worked:
tools.avrdude.program.pattern="{cmd.path}" "-C{config.path}" {program.verbose} {program.verify} -p{build.mcu} -c{protocol} {program.extra_params} "-Uflash:w:{build.path}/{build.project_name}.hex:i" -Ulock:w:{bootloader.lock_bits}:m
Thanks again so much! Cheers!