Apple already released a patch, version 9.0.1, which you must install after installing iLife ’11 and before you launch iPhoto ’11 for the first time, since that requires the iPhoto library to be updated. Find the 9.0.1 update in Software Update after installing iLife ’11.
Apple also published a support document containing more information, right here.
As always, you need to have a backup of everything valuable on your computer – and I reckon your photographs goes under “valuable”. So, why not check out Amazon for a 1TB external drive to do a Time Machine backup on? Otherwise I recommend CrashPlan as an online backup storage solution, which I also use for my 100 GB Aperture 3 library.