I just deployed a new version of the support portal which should fix the issue. As long as your ticket is still 'Submitted' (once it's 'Accepted' and being processed, this is no longer possible) you can choose to retract your ticket using the 'Retract ticket' button. This will change the status of the ticket back to 'Draft', allowing you to make modifications. However your ticket will not be processed until you resubmit it, which will return its status to 'Submitted'. (and makes it visible to processors)
I also turned off validation on tickets when changing their status to 'Closed'. This should solve the problems some people had, with attempts to close their old MXDN tickets triggering validation feedback on fields which are now required but were not in MXDN. (and were empty as result)
P.S. Remember to clear your cache, or you might not see the new 'Retract ticket' button yet.
Not being able to edit tickets after submission was a decision we made during the development of the support portal since we considered it not desirable to have tickets changed by the caller once they are being processed.
We'll discuss adapting things however such that submitted tickets can still be edited having the 'Submitted' status, before they reach 'Accepted' and beyond.