Therefore, subsequent revision of the earlier notice period can be done by the employer subject to the formal intimation of the change which seems to have been done by the poster's employer.
Having said so about the legal position of the effect of breach of notice clause, it would also be pertinent to discuss the prime objective behind the concept of notice of termination clause in the contract of employment. Notice of unilateral termination is primarily to inform the other party about one's intention of exit in advance so as to enable him to find the apt alternative. That's why it is very difficult to define the duration of the notice period very precisely. So the positive conclusion one should arrive at is even in the absence of mention of any specific period , it is necessary to give notice of exit.
Therefore, the objection of the poster with reference to the signed appointment letter can not hold good.
24th July 2019 From India, Salem
25th July 2019 From India, Jaipur
Here, the offer of the management is change of terms enhancement of notice period from 1 month to 3 months but not yet a contract as long as it gets agreed by the groups.
If the notice of change is not challenged/ objected shall be conditionaly treated as an acceptance.
25th July 2019 From India, Mumbai
28th July 2019 From India, Jaipur