firmware without cust pack

can i change my firmware, the fs and main, without changing the cust pack currently installed. or do i have to flash all three at the same time using a2 uploader.

currently this is what is installed:
C905_R1DA032_FS_America_Red53
C905_R1DA032_MAIN_Generic_Si_Red53
C905a_1212_2674_R6A_Customized_Generic_Lam_(b)

I want to change the FS and Main to:
C905_R1FA035_Fs_Western-Europe_Red53
C905_R1FA035_Main_Generic_Si_Red53

and leave the cust pack.

thanks for your reply

you can't leave custom pack behind when flashing FS, you'll get configuration error, so always flash custom pack when FS is flashed