Gathering requirements for a GUI application rewrite

 I have struggled with this for years (having been on 4 rewrite projects now) as a rewrite really does not fit into any process I have seen.  After much struggling, researching and thinking, I have come up with a plan that is working for me.  Many thanks to Ian Griffiths for his input on this:

1)      Four main lines of attack:

a.      Talk to the original system developers

b.      Talk to the users

c.      Use the system yourself

d.      Examine the code of the original system

2)      Document what the UI does:

a.      Write a section for each window/screen in the UI

                                                               i.      List all the things the window/screen can display

                                                             ii.      List all the actions a user can perform on this window/screen

                                                            iii.      List all the places a user can go to next from this window/screen, or all of the windows/screens that can be opened from this window/screen

                                                           iv.      List all of the ‘behind the scene updates’ that are out of the users control that happen when this window/screen is modified

                  b.      Compare the Lists in 1) with the functional requirements

                                                               i.      If its in the window/screen but not in the functional requirements, update the functional specs

                                                             ii.      If its in the functional specs but not on the window/screen, go back to the users

1.      Was it missed?

2.      Featured removed?

3.      Feature never should have been in the functional specs