Are the current & potential stats pre-determined?

As per the FM editor, each player has a rank between 1-200 in terms of how much of a decent player they'll become.

Can this estimation be changed during your game (without editing) or is someone like John Smith of Watford reserves always going to have a rank score of 68/200.

There has been some claims that PA changes for regen players depending on how they develop in age.

A lot of players usually have a value between 0 and -10 which will randomize stats and those values, but once the game starts its pretty much pre-set.

Pretty much everyone over a certain age (21?) will have a fixed potential. Younger players will have a potential within a fixed range determined by a number with a - prefix (ie -9, which gives a range between 150-180) - where the precise PA number will be fixed exactly once the savegame begins.

Newgens potential is fixed at the time of their generation into the game.