They will send you a new revolver. It is easier for them to do this.
They have not had available "blank" serial numbered frame for years.
They got audited by the ATF many years ago and penalized. No serial # leads to an untraceable firearm. As soon as a receiver/frame is completed in a firearm build-able form, it must be serial numbered. It is stored in that format wither it is assembled or not. Everything has to be traceable today.
Like anything else,there are exceptions,if your receiver was in current production,they could use your current serial number on a new receiver/frame. Not worth the extra cost, time, and headache for them.
They still have to have proof of destruction of your old receiver/frame to free up that particular serial number.