Hi,
I would be of the opinion that they can't make you over insure your home. Given the fallout of the property market labour costs, which make up a significant portion of rebuilding costs, have gone through the floor so surely the opposite, i.e. it now costs less to rebuild a home than in 2006, is the case.
The only thing to watch out for I'd say is the loan offer whereby it may stipulate that KBC determines the reinstatement value of the house at all times in the future.
But even if this is the case I don't see you having anything to fear if you have been truthful with your current insurer. If the worst happens and the house burns down which is then rebuilt from the proceeds of the buildings insurance policy then who are KBC to say that house hasn't been fully re-instated. No??