Do you have to check headspace with modern handguns, when switching barrels?
No, what toecheese said below.
no…and make sure you are using a “true” drop in or conversion barrel. For example, some people think that simply dropping a 9mm barrel into a .40 M&P pistol is GTG instead of using a true “drop in”. Just do some research and you will be rocking!
Depends on the platform your using. It will probably “drop in” but the one thing to keep in mind is the breech face width. That can affect your extraction and ejection. Sometimes its easier to get another slide in the caliber you want (given that it will fit the frame) You would still need to check and make sure your ejector will work with your “drop in” caliber choice.