I looked into this for Scotland, so it may not be the same in England and Wales. We pay an annual caution premium (security bond) with regard to my mum's Guardianship. The level of bond was set by the court in 2014. The company that manages the bond are not legally allowed to reduce the sum protected, even though my mum's estate has reduced. I was told we would have to go back to court to have an order issued to the cautioner for the level of bond to be reduced.
So, that's a long way of saying you need to ask the COP. The security bond provider should be able send you a schedule of premium charges against a rising scale of estate value. You will then see how much saving you could make by reducing the annual fee. In our case it was only about £250 and our solicitor said it would cost much more for him to apply to the Sheriff Court for a hearing in chambers to obtain the order. In England there may be a different procedure. Also, in our case, the Guardianship is only awarded for 3 years, so if my mum lasts that long our Guardianship will need to be re-applied for in 2017 anyway.