Before being able to perform the major update (from 2.3.5 to 4.0.4) you need to have the most recent software build - and that update can only be performed in Japan (not roaming) and on Docomo. The major update itself should not be locked to non-roaming and on Docomo but it might be. Either way, the only way for you to update to ICS is to send your phone to Japan.
In your case, you need to have build version 01.00.07 before you can try to do the major update.
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