1. It is not known that how the builder changed the name of the original owner by a registered Deed.
It can be considered as transfer of property, if so, then the original owners are deemed to have transferred their rights in the property to new person(buyer), thereby their rights in the property are extinguished.
2. Since the transfer is reported to have taken place by a registered document, you may peruse the document to form a clarity about the issue, you can refer the document before an advocate who will explain the details.
3. The apartment owners have already been protected with their rights for the undivided share in the property which would find a mention in their registered sale deed.
Undivided share is a part of land held by the buyer of the apartment in a residential complex on a plot on which the entire structure is constructed. Each and every flat built on that particular plot will have a share in the land but will not have any defined boundaries.
4. You have still not mentioned that what is the imminent danger or risk the apartment owners are facing by this transaction.
What is the kind of transaction is this?
did you go through the registered document to clarify your doubt?
Have you consulted with an advocate or discussed about this with the fellow apartment owners about it?
5. You can engage the services of a lawyer and et the documents vetted and also his opinion for further action if necessary.