I'm not sure that was the question the OP was asking...
If you mean for money then the starfiles say:
Code:
43N UPGRADE
44N --------
45N .
46N UPGRADE AT ANY TIME:
47N .
48N . * ANY ECONOMY ONEWORLD EXPLORER FARE MAY BE
49N . UPGRADED TO FIRST/BUSINESS CLASS ONEWORLD
50N . EXPLORER FARES WITHOUT PENALTY.
51N . * BUSINESS CLASS ONEWORLD EXPLORER FARES MAY BE
52N . UPGRADED TO A FIRST CLASS ONEWORLD EXPLORER FARE
53N . WITHOUT PENALTY.
54N . * AADVANTAGE UPGRADE AWARDS PERMITTED ON AA
55N . SECTORS USING AA AWARDS ONLY. /UPGRADES ON OA
56N . PARTNER SECTORS NOT ALLOWED./
57N . * STICKER UPGRADES ALLOWED ON U.S. DOMESTIC AND U.
58N . S./CA TRANSBORDER SEGMENTS ONLY.
59N . * AN UPGRADE TO A FULL MILEAGE RTW FARE IS
60N . PERMITTED AT ANY TIME WITHOUT PENALTY. REISSUED
61N . TICKETS MUST SHOW IN THE ENDORSEMENT BOX:
62N .
63N . NON-REF/XXX
64N . /THE XXX INDICATES THE ONEWORLD EXPLORER FARE
65N . BASIS CODE./
66N .
67N . * THE ORIGINAL NON-REFUNDABLE AMOUNT REMAINS NON-
68N . REFUNDABLE. IF THE UPGRADED TICKET IS
69N . SUBSEQUENTLY CANCELLED, THE ORIGINAL PENALTY WILL
70N . APPLY.
71N . * AN ONEWORLD EXPLORER FARE MAY BE USED AS CREDIT
72N . TOWARDS THE PAYMENT OF ANY HIGHER FARE TYPE FROM
73N . THE POINT OF ORIGIN PROVIDED THE RULES OF THE
74N . HIGHER FARE TYPE HAVE BEEN MET. REISSUED TICKETS
75N . MUST SHOW IN THE ENDORSEMENT BOX:
76N .
77N . NON-REF/XXX
78N . /THE XXX INDICATES THE ONEWORLD EXPLORER FARE
79N . BASIS CODE./
80N .
81N . * THE ORIGINAL NON-REFUNDABLE AMOUNT REMAINS NON-
82N . REFUNDABLE. IF THE UPGRADED TICKET IS
83N . SUBSEQUENTLY CANCELLED, THE ORIGINAL PENALTY WILL
84N . APPLY.
So upgrade is clearly no problem - although to make sense you would probably want to do it before starting the journey.
Downgrades (except of the type that JohnAx mentions due to lack of First on a route) are more problematic. I suspect that would be a cancel and reissue if it was before your first flight, and it muight be tricky to do after the first flight.
I don't understand why you would want to do this though? Chronic indecision?