If you own the firearms as an individual, the NFA firearms may be left in a safe deposit box in your former State of residence. The firearm can also be left or stored in the former State of residence at the house of a friend or relative in a locked room or container to which only the individual owner has a key. The friend or relative should be given a copy of the registration forms and a letter from you authorizing storage of the firearm at that location.
If you use a NFA Gun Trust, you can do the same as with an individual, but you may also leave them with another trustee or add someone as a trustee to leave the items with. You should be cautious whom has access to the firearms as they could create some additional liability on your behalf and those who are involved with the trust.