post and payscale changed in Offer letter than recruitment adv.in BEL

During my recruitment BELOP Pune subsidiary of BEL an govt. PSU.Recruitment advertise was mention post Technical assistant and payscale 5950-290-11920. I got selected in writeen exam,interview ,medical for post of technical assistant (i have call letter for writeen exam ,interview with name of post technical assistant.) but after document verification company give me offer for appointment on the post Senior Operator with pay scale 5240-180-8800 which is 7 year below post and payscale is also reduced without any intimation prior to joining or in recruitment process. as due document verification requirement i was resigned from previous employer i was have No option so i join on that post. is this legally valid what company had done with me i.e. change in post and payscale without intimation. can i challenge it in court. or what procedure to follow to get justice.