I often change my ming about the fact that a block is only for one sprite or not and this is painful to do.
I did not even find a way to move a piece of script from one definition to the other.
I know the might be problem with variables, and I may have to adjust this script.
you can change a block's scope (local / global) using metaprogramming in v9, but only if the block is not in use anywhere. Changing a block's scope leads to all kinds of unexpected and hard-to-manage conflicts down the road. It's almost always better / safer / more convenient to make another block with the other scope and select the instances to exchange / switch over manually from case to case.
Also, to tell you a little secret, global is mostly what you (should) want, whereas local is what you think you should want, but really don't