You will be governed by the terms and conditions of of emplyment as regards notice period for termination of probation. If the revised notice period for resignation ofprobation is incorporated/introduced after the date of your resignation, the cluase will not bind you and the employer is prelcuded from insisting you to giev revised notice period for resignation.
Any chnage/modification in the matter of service conditions inter alia cannot be retrospective
As you cannot seek to get relieved from probation by giving 15 days notce as against laid down period of 30 days stipulated by the employer that has been agreed to by you, the employer also cannot act aribitrarily in the matter of resignation notice period unless you have agreed among others in the probatioanry orders duly signed and received by you before commencment of employment
The emphais , however here is that the employer cannot insist you to give notice period in excess of what ahs been mutually agreed between paries ,more specifically the resignation for period of probation remians unchanged on the date of your resignation of probation
P.Senthilkumar