Another way to phrase the important point that christep made: tickets are good for up to 2 years: the first flight must be within 12 months of the date of purchase, and the last flight must be within 12 months of the date of first flight. The rules are at the time of purchase, so you can lock in the rules for up to 2 years. BUT you cannot make any changes to the first flight (either date or routing) without losing the lock on the rules; if you do that (and the change is free!) then the ticket is repriced at the current price (on the date when you make the change) and the rules are changed to the rules on that date (effectively you have refunded the original ticket without penalty and bought a new one). So in this scenario you must fly the first segment as originally purchased. All other changes, after flying the first segment, can be made without affecting the ticket rules or price (but incur a USD75 change fee).
Sometimes rule changes occur with no notice (for the 20 segment rule there was about a month notice). The airlines are paying close attention to the rules now during re-issue, perhaps due to some of the past abuses that have happened, but I have found the rules to be fair. And no problem in re-issuing a pre-20 segment ticket, adding extra segments (even purchasing extra segments for a continent), after I pointed out the original issue date.