There are some posts from a year ago, but no concrete details.

I think it's possible. See page 6 of this pdf on MtGox: https://www.mtgox.com/img/pdf/201504_notice1_en.pdf

Post-Filing Change of Details of Exchange-Related Bankruptcy Claim Which You Filed Using the Online Method In the case where, for example, your address has changed after you have filed your Exchange-Related Bankruptcy Claim, the relevant details can be changed using the Online Method. After logging in to the System, please click the “View or change the details of the bankruptcy claim that you have filed” button, and change the details. However, regarding any changes to the creditor’s name which arise due to a change in the right holder of your Exchange-Related Bankruptcy Claim (pursuant to a transfer of claim, etc.), please see section IV.5 below. In addition, please note that it will not be possible to increase the amount of the bankruptcy claim which you have filed after the deadline that the bankruptcy trustee will announce at a later date

In case you want to change the rights holder (from you to someone else), it's more complicated, read the section following the one i quoted, it's on the same page in the pdf (page 6).

no idea how to do this though ...