Discrepancy in Flat construction and Property registration

Sir, I have purchased one Flat from ABC developers and we have agreement and registration between me and developer that I am purchasing Flat XYZ. Initially developers send me demand letter for Flat XYZ and after 1-2 payment later they sent me Demand Letter for Flat ABC. I imediately reported to the developer and asked them to make the correction and send me the Demand letter with correct details. They said due to some issue in CRM software due to this happened and they said they will get this corrected. I have taken follow up but they always had excuse that they still working on this. Now when the property is nearing to possession and i visited the site to check the Flat which i purchased. Then, i was surprised to see that my name is written in name plate and letter Box of Flat ABC rather than Flat XYZ. I have reported imediattely to the developer that i have registered for Flat XYZ where is my flat but they are saying due to change in structure layout Flat XYZ is scraped and you will get possession of Flat ABC. Now the question is that how i would be the owner of Flat ABC which is not mentioned in Registry\Sale Deed? In future if i am going to sale my Flat then how i can sale it. Please suggest all best possible option i have so that i can't loose my hard earned money and get possession of my Flat. Regards, Shankar