Is it possible to buy/upgrade the nozzle on my Ender 3 Pro to one that is suited for handling the higher temperatures needed to effectively print with PETG filament?
No hotend upgrade is required to print PETG on an Ender 3 (pro or any other variant). The stock hotend can be used at temperatures up to 250 ˚C, and 230-245 ˚C is the range typically recommended for PETG. I print it at 245 ˚C.
You may however want to upgrade the extruder. The stock extruder lacks both torque and grip, and significantly skips/slips when trying to print PETG at any significant speed. Going over 40 mm/s or so is likely to produced failed prints. Look for an extruder with gear reduction and a hob that's designed to grip the filament better (vs the flat one that comes with the printer) or even dual driven hobs.