When I swap out one of my original 0.4mm hotend with a new one, is there any calibration needed to be performed before using the new hotend or do I just start printing?
You definitely need to re-calibrate nozzle length and position.
A flow calibration could be handy, but is not required in my opinion.
Though the exact diameter of the nozzle might be different, it doesn’t influence the amount of material passing through.
you need to redo bed-mesh and nozzle offset. you can also redo input shaper but usually that would not matter too much.
Care to elaborate on that? Which calibration option would that be?
Tbf, I’ve never done any re-calibration when switching to 0.2, 0.6 or 0.8. Prints all came out fine.
It should run hot end offset calibration automatically before your next print when it detects a different size in printer. If you want the best accuracy you can run the precision nozzle offset calibration with light and dark filaments.
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Your new nozzle doesn’t necessarily have the same length as the old one (worn down) and the exact XY location of the nozzle might be slightly different, which becomes visible when you are printing with both nozzles on one part.
I know that the H2D does a lot of pre-print checks (including length) but they depend on your printer’s settings. The relative positioning of the 2 nozzles definitely requires a separate calibration with 2 contrasting colors so the printer can optically measure (great automation by Bambu btw.) and adjust that.
You might been lucky with your set of nozzles, but results in the past are no guarantees for the future 
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I see you already answered the question of @simtom also 
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Yes. Also the design of the hotend seat made it not 100% calibrated and vertically aligned. This would also cause some issue if the user don’t recalibrate the offset. If you don’t print with the left nozzle on first layer, the problem would be rather small. However a dual nozzle first layer definitely requires nozzle offset + double bed mesh.