Assuming that the mechanoid's head contains it's personality (which seems logical given when Kryten visits his heads they are all capable of interdependently talking and conversing with him). What happens when he uses one of his spare heads instead of his own?
Does he stop being Kryten and start being Spare Head 2? Is Kryten's personality written over the Spare Head's?
Is there an episode where we see a Spare Head being used as his head we can use to establish this?
Yes, such an episode exists - Tikka to Ride.
To summarise from the wiki page, Lister is distraught that all the Indian food supplies have been destroyed, including all the poppadoms. He wants to return in time and pick up another 500 curries, something the other crew members are against.
As he isn't winning the argument, he waits until nighttime and swaps Kryten's head for one of the spare heads (removing the guilt chip from it). From the wiki:
The next morning, Rimmer and Cat are confused when Kryten serves them high-calorie breakfasts, starts smoking and using phrases like "you bet your ass". However, when Kryten assures them it'll be ok to pick up the time drive the two assent and board the ship containing it again intending to go back to the 21st Century to one of Lister's favourite curry houses to pick up a large order.
So by swapping the head (and removing the guilt chip), he is able to get a guiltless Kryten. However, it's not just the lack of guilt that has changed, but also his personality.
Remember as well that when we first meet the Spare Heads, in DNA, they refer to him as you and we see all their distinct personalities on display.
So in summary, each head has its own personality and swapping them for his usual head does cause personality differences.
