There would not be any objection from any side if you opt for mother's cast to be retained for all your official purpose. You need to obtain a declaration from your mother that she is involved for bringing up you meeting all your financial expenses apart from looking after all your activities. This statement has to be certified by the Mukhia or Gram Pradhan of the village and in case of living in a town area, you need to contact S.D.O regarding the authenticity of such a declaration.
Once this formality is over, approach the lawyer of your local court to get issue resolved. An affidavit would be required for the change over of your caste. With the guidance of the lawyer, you can have an affidavit certificate indicating your caste status in the general category. Get the same advertised in two papers - one in local and the other in prominent English Newspaper and preserve a cutting of such a notification for the production of the same to be shown to the verifying authorities in case of need.
However, before executing the same for your benefit, you should examine the pros and cons of the benefits which you might have got by retaining the caste status of your father belonging to ST. While seeking for an employment in the job both in the central government undertaking or in the private, you would enjoy the concession in terms of marks, age and other benefits prescribed for a job for an aspirant of ST. You would lack such facilities being a candidate of general category.