Interesting thread. @:-)
Lets say I am on a fully-changeable Y-ticket LAX-ORD-FRA, original ticketing date 11/14/18, original travel 1/14/19.
On 1/10/19 I change the routing & date of my ticket to LAX-IAH-FRA for travel 2/14/19.
How long is the changed/reissued ticket now valid for?