B.a. to m.com

No you cannot directly do Mcom after BA. You will have to clear commerce subjects ie Bcom to go for Mcom, Atleast this is the Rule for Mumbai University which I have mentioned here.

If you are a Science Person you can go for a change to Commerce or Arts in continuation at the next level. 

If you are a Commerce Student than you can Convert to Arts stream at any level in continuation for further studies.

If Arts then you cannot directly convert to other Stream in continuity.

Check the Mumbai University site