I have never heard anything so unlikely - it's more likely that they are trying to charge you an admin charge for the change to the insured names on the policy documentation.
I used to work in insurance and the risk for household and building cover was based on the postal address, proximity to water, earthquake risk, and so on.
Like you I have had different occupancies in my house - I lived with someone, we split up so only one occupant, some years later someone else moved it. The names on the policy changed but I never had to pay extra for that privilegel
I would shop around, you may not wish to say which company you are with at the moment, but there are plenty of others - you have no obligation to stay with the same company. We change our insurers most years when the renewal comes round anyway, to get the best deal.