If I understood your narration very well, I wouldn't be too warried about who the new person is behind the telegram account. I am more concerned about the deposit that has been made to you. Are you going to inform law enforcement agencies and give them the money so that they can carry out an investigation and where possible trace and return the money to the owner since the job was not done? Just block the new person and the problem is solved. Whenever the old person resurfaces, they can always reach via your telegram handle.
I don't think that money is the issue here, it was a upfront payment to the OP, so obviously he did the job and just waiting for the rest of the money for the completion. However, the person that he talks to Telegram suddenly disappear and then resurface as some time with a new name and that's why he is asking if it is still the original person that he talks that deposit $2,500 or a different one.
If it is a different person so when the hell does the original person that contacted him and give him the job and the downpayment?
So it's really hard to give advise here, we can all speculate as to what happen to the original person (I assume the OP thinks that hew as kidnapped?). Or it is still the same person, why he went silent after half a year and it seems that he is not concern with his $2,500 or if the job has been finished as agreed and on time?