The letter from last year said to return the plates "as soon as possible."

While in some contexts that means "immediately," taken at face value it can mean, "at your earliest convenience."

Either way, "as soon as possible" is not a specific deadline, and it is therefore too subjective to be unenforceable.

Whoever worded the letter screwed up.  There should have been a date or period of time (30 or 60 days) to have the plates returned.  Can't hold anyone accountable for an ambiguous phrase.

As I said before, I was able to keep my vanity plates when I opted to have new regular plates issued.  They ought to do the same before some agent of the state comes and removes them.  At least they'd have a great Hawaii souvenir.     

I lost my plates in a boating accident...

I would say "FCKC&C"