Easy solution is already available:
(1) Save the message
(2) Leave the submit menue without submitting
(3) Enter afresh via the 'Start new thread' option
(4) Enter the title you now want to appear
(5) Paste the saved message
(6) Check title to see if it fits your present state of mind
(7) If yes, hit submit
(8) If no, go to (1)

This is a solution certainly. I am not sure that it is 'easy' or the best one........