Change whether an existing custom block / variable is "for this sprite only"

Sometimes I make custom blocks or variables without fully considering whether they ought to be "for this sprite only" or not. After I've created it, there is no way to switch it to the right scope! This is very inconvenient because it means I have to recreate the block/variable entirely, replacing all existing uses of it and recreating the labels/parameters if it's a custom block. Multiply this by however many custom blocks I want to change, and it becomes quite a pain!

Obviously, if a custom block or variable is already used outside of the selected sprite, the option to make it "for this sprite only" should be disabled.

Yeah. We want this, too, but Jens says it's hard.