I saw on
http://modmyi.com/forums/3g-s-downgr...phone-3gs.html
That there is a spesific way for upgrading to 3.1:
" Alter the host file as described by Saurik and use his method to restore your phone to 3.1. Doing this will store your ECID with Cydia for future use. "

What is that way?

Will I be able to downgrade bak to the older version?

Thanks.