Seat upgrade is impossible if the stadium itself can't be upgraded. At max level stadium, there is no seat upgrade.
Seat upgrades are actually partial upgrades for stadium upgrade. After completing a seat upgrade, the cost, build time and effects for further upgrades will decrease accordingly. If you login daily it is best to do the smallest seat upgrade first as the new seats will be available eariler, while the total cost and time needed to upgrade for the next stadium stay the same.
Upgrading directly to next stadium will complete all the seat upgrades not built yet, but you need to wait longer before new seats become available.