You're not locked to any certain reward actually. You can donate whatever amount and choose whatever reward tier up to the maximum amount you paid.There aren't even enough rewards tiers to really do that anyhow. They'd have to buy like 600-700 $125 tiers and and buy out all the high-level rewards, since everything over that limit has a hard cap. Logistically it would be very very hard to do that in a way that doesn't look like fraud. You'd need 700 different Kickstarter IDs, 700 different Amazon accounts, 700 different credit cards.
I found this out when I raised my funded amount of three kickstarters from $1 to $40~50 and saw that I didn't actually choose a reward tier. :\
edit: SO MAKE SURE AND CHECK THAT YOU'VE CHOSEN THE NEW REWARD TIER YOU BACKED.