[SOLVED]New bie confused on changing or deleting plugin after composition.

Hello , I try to learn Qtractor and using in my projects enjoying and admiring. In my last project i used some plugins and record sound thru my MIDI keyboard. To do this i created a separate midi tracks and buses and used a synth (ACE fluintsynth) and a reverb (Calf reb) plugin in the busses. In the synth plugin set sf2 soundfont (Yamaha c5..) evrything was fine but afterwards i didnt like the piano sound and decided to refine it. The problem is when i change the plugins even delete all the sound doesn't change . I understood that midi tracks are just digital codes and consist some info about pitch, duration etc. I expected that when i deleted the plugins it should be no sound. I searched the plugins elsewhere in case of i missed to delete but no way. Im sure I miss something , would you please help me about it? Thanks.

Edit : VMPK was set Sonivoweas synth and qtractor was connected VMPK , because i need to show the keyboard. I set the VMPK 's driver and connection to Fluitsynth , im happy now.