colchiro
Extreme Android User
Here's how I upgraded from rooted GB 596 to the GB 602 and retained my data. 
Obviously your mileage may differ. This assumes you are rooted and on a rom based on the stock rom. Not CM7, OMGB, OMFGB, MIUI, etc. Users of non GB roms should not restore data, unless they're prepared to take the risk.
Not rooted? (stock GB) You can't make the recovery or Titanium backups and can't restore data, b/c you don't have root. You can still use this guide to upgrade, but just the first section. You will lose data, but your apps can be downloaded in Market.
Running some other rom? (not on a GB-based 596 rom) You probably shouldn't restore data (second part of this guide), but can still use this guide.

Obviously your mileage may differ. This assumes you are rooted and on a rom based on the stock rom. Not CM7, OMGB, OMFGB, MIUI, etc. Users of non GB roms should not restore data, unless they're prepared to take the risk.
Not rooted? (stock GB) You can't make the recovery or Titanium backups and can't restore data, b/c you don't have root. You can still use this guide to upgrade, but just the first section. You will lose data, but your apps can be downloaded in Market.
Running some other rom? (not on a GB-based 596 rom) You probably shouldn't restore data (second part of this guide), but can still use this guide.
- Copy z4root and d2bootstrap to sdcard. Google them and download them if you don


)
I was a little surprised that it worked but it did.