The rules as published by CX (see here:
http://www.christen.demon.co.uk/CXRule/ )
are clear:

Rebooking

Before departure: 1st coupon - permitted (reissue ticket); other coupons - permitted (validate ticket)
After departure: other coupons - permitted (validate ticket).

Reissuing the ticket causes the new rules to be applied, revalidating it does not.

The CX Rules do not explicitly forbid or allow open dating.

The * file here: http://www.hardlink.com/~markdu/OWFi...eExplorer.html

says:

85N RESERVATIONS
86N -------------
87N .
88N RESERVATIONS FOR THE FIRST INTERNATIONAL FLIGHT, AND
89N ALL PRECEDING FLIGHTS, ARE REQUIRED AS FOLLOWS:
90N .
91N ------------------------------------------------------
92N I ORIGIN IN AREA 1 I ORIGIN IN AREA 2/3 I
93N ------------------------------------------------------
94N I 7 DAYS I ANYTIME PRIOR TO I
95N I I DEPARTURE OF THE FIRST I
96N I I FLIGHT. I
97N ------------------------------------------------------
98N . * ALL OTHER FLIGHTS MAY BE OPEN.


and

106N PRIOR TO DEPARTURE
[...]
145N TRAVEL ORIGINATING AREA 2/3:
146N .
147N . * UNLIMITED CHANGES PERMITTED WITHOUT CHARGE.


Debates such as this continue to convince me that there is no value in dealing with Travel Agents on ONE tickets unless you can find one who is clearly expert. Why not just deal directly with your local OneWorld ticketing office?