Heres a guide for upgrading to 2.3.6 form any firmware..This guide is ONLY for SGS GT-I9000..
This guide can also be used if your phone has a boot loop and you cannot access CWM Recovery..(Provided your Phone Boots atleast to "Galaxy S ...
I wanted to create this guide as I'm seeing allot of SGS users bricking there phone..This guide works for SGS GT-I9000 only..
ALSO USE THIS GUIDE IF AND ONLY IF U SEE THE FOLLOWING IMAGE WHNE BOOTING YOUR PHONE(attached image:1)
You can flash 2.3.6 form the link below..
First of all you need to upgrade to 2.3.6(link given Below)..then root it..
then install the update file like CM10 for jellybean..etc..
No issues...just downgrade to 2.3.6 using odin (Link Given Below) & root it..
Then flash CM9..reboot..
after the first boot goto CWM using the 3-button combo and restore from your nandroid backup..