I get the problem, but...
Short answer: it is not doable.
A bit longer version: individual map cells ("polygons") are not independent, they affect their neighbours in multiple ways depending on their content, so it's not possible just to replace one cell with another. In previous versions, something like what you propose could be implemented (here is a similar idea), but not anymore.
Maybe in the future I'll find a way to make local changes, though.