I always recommend people keep small upgrades going until the arena is ready for the next highest division. One of the things I notice is that most teams only seem to build the arena they'll need for the EBBL once they've already gotten here. The ones that already have a fully built arena can immediately start taking advantage of the increased income potential, while the ones that don't are scrambling to catch up and typically are the ones that go down right away. They're forced to spend the promotion money on an arena rather than the needed upgrades to their roster to be competitive. So small, constant upgrades of 100-300 seats is an easy and productive way to improve your clubs future.