You would have to start over. This is because w/ features added/removed, the files wouldn't translate w/o corrupting data.
The most obvious example I am aware of would be the badges. 2K added and removed a lot of them between current & next-gen. Any added/removed signatures etc would likewise throw a roster out of wack. You'd have to spend an exorbitant amount of time cycling through each player to restore them anyway.
2K updates contracts as the info becomes available. In 2K21 they (finally) fixed a multitude of errors to put them more in line w/ accessible cap sheets, which would indicate they will probably update as the official data becomes available.
As far as I can tell, the salary restrictions are built into the individual players. I believe they do this because IRL the NBA limits player earnings based on professional seasons the player has played (i.e. the max for a 4-year pro is different than the max for a 10-year pro).
My assumption for why they do it this way is to prevent 4-year players from getting salaries in the 50M range in MyLeague. In effect, it's a workaround for coding that would otherwise need to be much more intensive to make work properly.
The problem is 2K's salary limits haven't scaled WITH the escalation of the cap. Most of the developers are not well-versed in the technical minutia of the NBA, and probably don't even realize this is an issue.
This is primarily because 2K caps their Ratings and Tendencies at 100. Harden (and most other great players) break the scale in quite a few areas. Harden in particular should have a Shot Tendency of 110 and a 3PT Tendency of 120. (This also doesn't factor the sim engine doesn't generate enough FTs for volume scorers; Harden averages 12 per game).
It's important to remember the core of 2K was developed and programmed in the late 90s. Principles in the NBA are obviously very different now. Teams had broader usage back then and the engine likely wasn't equipped for the game to become so intensely star-centric (or for the mid-range / post game to be completely abandoned).
The jump to next-gen has no bearing on these areas. Technology isn't the limiting factor in how the game is programmed at a base level.