I've come across this a few times with functions and haven't found a good way of fixing it, other than to manually copy the whole thing over. I have quite a large function which I thought would require a return value, but realised later that it didn't. I want to change it to not require a return value but I don't know how.
And just to add, I tried removing any calls to the function but it still doesn't want to let me change it. If I'm forced to change it manually, is there a better way other than line by line copying it?