Re: NCC = Not Constitution Class?
-the Jefferies system does not have to continue into the 24th century. A switch to straight serial numbering could have occurred as the Jefferries system became too cumbersome to use. Such changes happen all the time IRL.
That's the way I've generally looked at it. I do think Starfleet underwent a few changes between TOS and TNG, including what classifies as a starship.
"Don't sweat the small stuff--it makes you small-minded..."