I don't doubt a change of address form for US mail can be completed online these days. Much easier than it used to be. But to change an address, the person needs a new address to change to. Is the assumption G&B were going to live with Gabby's mother and stepfather next? It seems odd that was never mentioned by her family. And odd NS seemed to think all of GP's property was in FL.
When I questioned this idea previously, posters pointed out that young people (as well as other people of any age) may do things differently than I would. I'd certainly agree with that. But I still fail to see why they'd try to become NY residents before their nomadic trip while maintaining the van registration in Florida. It wasn't for insurance purposes as car insurance costs more in FL than NY. And so far as I know, a person doesn't really have a (legal) choice about where to register a car. It's registered in the state where one lives. Finally it wasn't clear to me that either G or B intended to get paying jobs anytime soon. But NY does have state income tax and FL doesn't. Looks like a 4% tax rate on $0-$8500 if filing single.
New York State Income Tax: 2021 Rates, Who Pays - NerdWallet
Why sign up to pay that without living there? Just seems weird. And like
@CharlestonGal said, it does seem the NPPD may have evolving reasons for not acting sooner.
If GP and BL wanted their US Mail to go to GP's mother for some reason while they were traveling, they could have had it forwarded. They didn't need to "move to NY" for that to happen. Just odd. And odd if JP moved to FL to be close to GP as he said he did.
JMO