1) Submitter A submits Item A into Project A. The Submitter is captured by Contact Record subrelational field 'User Name' (first last).
1a) Other times, Submitter B submits Item A on behalf of User A. In this case, Submitter B selects User Name from its drop-down list.
2) User B submits Item B with a subtask transition) into Project B from Item A.
2a) Item A's Submitter is the appropriate owner of some states in Project B--not Item B's Submitter.
3) The goal is to change the submitter for Item B, therefore, to User Name, assuming it's always a match. Speaking of that, there must be a script way to obtain the correct Submitter name from the Contact record, so that I'm not scripting a mechanical copy.