This is a paraphrase and I am not a competent person (that's too high a hurdle), but I think it will come down to the "reality" of the situation versus the rules going forward. I have held a DOT number.
DOT mishmash applies to people engaged in commerce, using vehicles over 11,000# GVWR if my memory serves me right and perhaps going over state lines or more than 150miles or some combination. The DOT rules can be very hazy if you've ever looked at their crazy rule book.
Transporting hazardous materials while conducting business, which I believe includes compressed air, O2 or most anything compressed means you need tanks, trucks and drivers that comply with all the rules. You could not use them in business that includes heavy trucks. Which would probably mean a commercial dive operation with some big trucks that had a DOT number would be out of luck.
For us it should not apply to transporting them.
Getting fills would be dependent on the shop. If you have a hydro with 5 years I would imagine that you would get a fill. The question is, down the road, would a hydro test operation risk putting themselves in some weird position by passing an out of compliance tank and then having it wind up in commercial service. If you can keep getting them hydro'ed I imagine that you'd find plenty of willing dive shops to fill them going forward.
There are plenty of people who fill out of hydro steel tanks, there are not as many people who will fill an aluminium tank out of hydro. It is always handy to know someone with a compressor.
-matt
