I repeat here an answer from Rong Chen:
Currently, GraphLab only supports ADD new data (vertex or edge) to the graph.
You can load new data as normal (e.g., graph.load_format()), and call graph.finalize() again before computing on new graph.
But, you CANNOT change the partition method and remove data (vertex or edge) from graph.
About graph partitioning, if you use random or grid method, there is no problem to partition new data.
But if you use oblivious method, since the heuristic information has lost after finalizing, so the decision on new data maybe bad.