To be fair, Jiang Lu could easily transform into An Dan's appearance and use Fake Voice now; it would be a piece of cake for him at this point.
But he chose not to do so. The reason being, he couldn't accept becoming that fat; it was a great blasphemy against the self-discipline and fitness he upheld for so long.
Moreover, he had attempted to act with such a physique before. In the Cell Factory, all of his activities in the Netherworld were carried out using An Dan's body, which left him with a great psychological shadow. It can be said that once bitten, twice shy; even though he knew it would be much more convenient, deep down he still resisted becoming a fatso again.
However, whether he did it or not didn't really make much difference fundamentally.
