Change in Company's policies related to notice period

Hi, In my company there is a probation period of 3 months, after which we are supposed to get an Appointment Letter. Its been 6 months since my joining and till date I haven't received my appointment letter yet. We have to serve 2 months of notice period once our employment is confirmed with the Company, however, we are only supposed to give 15 days notice period when we are on probation. Now, the company is changing the policy by just sending us an email stating that they will take 7-10 days time to accept the resignation and thereafter every employee irrespective of him being on probation will have to serve the company for Minimum 6 months and no resignation will be accepted before that and they will still have to serve 2 months notice. I want to resign as per earlier policy as my employment is not confirmed by the Company by giving an appointment letter. I went through a few laws but could not find any provisions related to this. Any answer would be much appreciated. Thank you!