Arrange: Choose Instrument Sound First, then change Patterns

Why isn´t it possible, when i have choosen the Main Track Chord Progression and then want to add a Bassline (Expression), that i add the Basstrack, then choose the external Sound from a Synth and THEN go through the patterns? The patterns can only be judged, when they are played by a fitting sound? Can i change the patterns afterwards and ca keep the created Bass track with the choosen Bass Soud?