I closed in 2009 on my first home, but I didn't get the full amount of the tax credit. It was only $6300 because I earned more than the current $75,000 income limit. But now that the income limit is being raised to $125,000 for an individual, I'd qualify for the $8000 tax credit. Can I reapply?

No. The new rules only apply to purchases that close after December 1, 2009. You have to think of this version of the $8000 first time home buyer tax credit and $6500 trade-up buyer tax credit as entirely new tax credits that have nothing to do with whatever came before.