[jntracks]

Just means make sure your midi performance is correctly pasted into the new instrument track and playing back correctly before you delete the other track. in case something wasn't right, you wouldn't want to have lost the midi data (performance).

Alternatively you could just option click on the tracks you don't need anymore and say "hide and make inactive". That way they aren't visible or taking up resources in your session, but if you ever needed to return to them later, like if the midi data was lost or messed up in the transition, you could do "show and make active" and you're back to normal.
Rather than actually deleting them, which is permanent.