What is the major hang-up of doing the upgrade?

If the sensors do not come out in one piece. This applies to the Temp Sensor cartridge and sometimes the Heater cartridge.

In 10-20% of the cases users are unable to get the temp sensor (3mm diameter) out of the standard hot end. By pulling its cables it is destroyed.

In fewer cases the heater cartridge (4mm diameter) is also destroyed.
EXPERTS will still have a good chance of recovering the cartridges, by carefully drilling a 2-3mm diameter hole from the opposite end, of maximum 4.95-5mm (varies) . This will enable you to push the old sensor/heater out, if it is not destroyed by the drill bit.

The cause of this predicament is a manufacturing glitch in the hot end / sensor size. It is also caused by the hot flooded by leak because of a collapsing Teflon piece and broken Steel coupler (the grey one with the tiny radial holes). If the hot end is allowed to print or stay hot for some time (minutes or hours) the flooded filament is carbonized and gets very hard.

The cartridges will not be recoverable in such cases.