She can call herself whatever she likes, though presumably she also intends on changing her name legally. The Beale thing would make sense to me if she only had Ian (or if Ben was also a Beale), but as things stand it would be more diplomatic to go back to her maiden name.
SOC obviously just wants Kathy Beale back in all her glory

(and in fairness she's much improved lately, and putting her back in the cafe was a good move).