Nothing, but it won’t make any difference.
The prebate represents a reduction in your tax liability. The only way you benefit from the amount being based on your 2007 income is if your 2008 taxable income results in a total tax liability of less than what you got a rebate for.
It’s complicated to explain without drawing charts. I tried to type it out here, with the 2001 rebate: http://boards.straightdope.com/sdmb/showpost.php?p=9460128&postcount=9
Consider the following analogy:
You want to buy a computer, so you’ve been saving money up for a year. You figure the computer will cost about $2000, so you’ve been putting $200 a month away for the year.
So, at the end of the year, you go to the store with your $2400, ask to get the computer, pay the $2400, it only costs $2000, and get $400 back in change (aka “refund”)
Or, you go with your $2400, but it costs $2500, so you have to pay an extra $100 not from your account (aka you “owe taxes”).
Now, consider in the middle of the year, the manufacturer looks at a survey they sent you last year where you said you might like to buy a $2000 computer this year, and sends you a $1000 refundable coupon for any computer they make, if you get one less than $1000, you get to keep the change.
But the process for spending the coupon is to cash it in for full cash value at one desk before buying the computer at another.
So, at the end of the year, you take your $2400 to the store, cash in the $1000 coupon, buy the $2000 computer, and have $1000 from the coupon and your $400 change - $1400 total (“refund” plus “prebate”)
or, at then end of the year, you buy the $2500 computer, cash in the $1000 coupon, so you paid $100 extra, leaving you with $900 total (“pay taxes” plus “prebate”)
or, after getting the coupon, you decide to only save $175 for months 7-12, so you take $2050 and the coupon to the store, and leave with $50 in change and the $1000 from the coupon - $1050 total (“refund” plus “prebate” with adjusted w9)
or, you decide you don’t want but maybe a $500 computer and switch to saving $50 a month. The computer company uses the survey they sent you last year and sends you the $1000 coupon anyway. After you get the coupon, So you go to the store with the $600 you saved up, plus the $1000 coupon. You buy a $600 computer, and keep all the the money from the $1000 coupon, but they guy at the cash desk tells you that had they known you only wanted a $600 computer, they would have only sent you a $250 coupon (“changing tax brackets” and keeping the “prebate”)