You can IF the insurance company will LET you. I'm not kidding!! That's why I canned my last insurer way back when....I wanted to decrease my value. We were at $350,000 hull value which was way over value, but that's what the survey said that the insurer had in hand. The insurer boldy told me that they would not insure for anything less "because we don't want to lose any money on the premium." I told them to go pound sand and for that comment they lost the entire premium.
I am in the same boat again right now with a policy renewing August 3. My $10,000 premium did not increase, so I guess I should be delighted that I'm not paying $20,000 this year and that it's still $10,000 and that's for $300,000 hull value.
I am trying to get a premium quote from my current insurer with an agreed hull value of $225,000 or $250,000 to see what that does to the already outrageous premium.
Bottom line...my experience with more than one insurer has been that they do not like to reduce values in order to reduce premium, but I am wishing both (all) of us good luck on this one. I'll let you know how mine turns out.