I think OP is referring to the American system where upgrades are given when space is available in premium cabins rather than the European/Asian/M Eastern system where upgrades are only given when paid for (cash and/or miles) or when Y or J class is overbooked...
So to answer his question: no chance to get a free upgrade, unless you're I/O, Plat, Gold+, Gold, or Silver and J is overbooked...