When you look at a players card.....Red equals current value and blue equals possible potential.
General attributes (strength, speed, acceleration, intelligence and discipline) rarely change for drafted rookies. Possible +/-1 change can happen but for the most part, what you see is what you get.
After drafting a player, general attributes like speed, acceleration and strength will change if you move them to a different position that will gain/lose them weight.
DEFENSE1 level jump
DL to LB lose weight....speed increase
LB to DB lose weight...speed increase
2 level jump
DL to DB lose weight...speed increase
OFFENSE1 level jump
WR to RB gain weight...speed decrease
RB to FB gain weight...speed decrease
FB to TE gain weight...speed decrease
TE to OL gain weight...speed decrease
2 level jump
WR to TE gain weight...speed decrease
FB to OL gain weight...speed decrease
WR to FB gain weight...speed decrease
RB to TE gain weight...speed decrease
3 level jump
RB to OL gain weight...speed decrease
4 level jump
WR to OL gain weight...speed decrease
If you go in the other direction, its the exact opposite. Each level you jump increases the amount of weight lost/gained which effects speed