Make a copy of the email(s), write a formal letter to HPD explaining the situation and either mail it in or drop it off. You could have it notarized and sent registered mail, but I think that's overkill and I wouldn't do it.
You can't apply for a permit for the replacement pistol until after the FFL registers it with HPD.
You shouldn't have to explain anything to HPD. Just give them notice the old gun is no longer in Hawaii, and go through the permit process for the new one after the FFL gives you an invoice with the handgun specs. One gun out, another gun in. How you came about getting the new gun is irrelevant AFAIK.
There are examples of what info to include in a transfer notice letter. The requirements should be similar for a gun permanently leaving the state. You can either send a notice letter via REGISTERED MAIL to HPD, or hand-deliver it to the same records window where you pick up completed permits -- right next to the table for COVID screening.
I tried to attach a PDF form I created from the transfer notification form I have, but the forum site appears to have a full storage area.
The only changes to the new form were the Subject line, the statute reference, taking out the transferor/transferee fields to make just owner fields, and adding an effective date, since this could be turned in prior to you leaving with the firearm.
To use the form in Acrobat Reader, open the PDF and then select the "Fill & Sign" tool in Acrobat. As you click on the existing blank fields, a text box will appear. Click in the box to enter your information. When done, you should save it as a different filename and print/sign it. If you prefer, just print the blank form and complete it by hand.
I'll try other options to get the form posted...
