At present if you use pathauto and your path changes, the old graph will not be removed because it will try to delete the new graph name which does not even exist yet.

The solution to changing graph ids is a unique URI for each entity. Fabien Gandon had suggested at SemTech to maybe use a specific (dereferenceable) URI for graph different from the node/entity URI. At least, we should use the entity id instead of the url alias. An alternative is to use the URI based on the uuid.