[UX] Allow reusable blocks to be made non-reusable (by cloning them into a non-reusable instance)

I was looking to improve the UI text for the "Make this block reusable" setting (in #2731), and was baffled by this bit in the help text:

A reusable block can never be converted back into a stand-alone block.

Now, I know what the differences between reusable vs stand-alone custom blocks are, so I fully understand the above, but why are we preventing making a single instance of a reusable block back into a stand-alone one? So for example:

• User edits existing reusable block in a layout.
• We have some help text to explain that a) this is an instance of a reusable block and b) making changes here will affect all instances (copies) of the same block, across any layout it may be being used in.
• We have a "Convert to stand-alone block" checkbox.
• We explain that converting the reusable block to stand-alone will "break its link" with the "source", and that it will then be possible to configure it independently.
• During validation, we check to see if the user has ticked that checkbox, and if so, we then save the same block properties (that are in config for the reusable block) into a "regular" content/stand-alone block.
• We finally update the config of the current layout, to replace the position/region of the instance of the previously-reusable block, with its new stand-alone "clone".

That should work right?