Can I change the spec of a MyBlock If I create my own block (MyBlock) with one param and later discover that I should have used 2 params or different icon or something like that, what do I do?

Is it possible to change the existing definition of a block or do I need to build a fully new block?
Just start a new block with an operation block you won't need, set the proper number of params; select everything in the old MyBlock and paste it into the new, then delete the thing you don't need.