Gail and Beth are both right.
The old tickets that were purchased prior to MGM and AK opening do in fact say on them in varying language that they are either not good for parks opening after such-and-such a date or they are only valid for admission into MK and EC.
For MGM, Disney would charge a premium to upgrade those old tickets to include admission to the new park. In MGM's case, the tickets had to be physically handled and stamped by the gate CM. If they found anyone using old tickets prior to MGM opening, they sent them to the Guest Relations window to upgrade or buy new tickets. This lasted until around Thanksgiving of 1995 when Disney switched to the ATS ticketing system. At that point Disney decided that the amount of pre-MGM tickets still in people's hands wasn't worth the time, money, and effort of reprogramming of the ATS database to exclude them from a single park while allowing them into the other two. So at that point, Disney started allowing any ticket to be used at MGM regardless of date of purchase and regardless of parks operating at the time of purchase.
They did a very similar thing when AK opened in April '98. From opening day through January of '99, they charged a fee to upgrade old tickets that were purchased prior to AK's opening. But due to the large gap between projected AK attendence and the actual number of guests visiting AK, Disney dropped the upgrade fees and started allowing any ticket admission to the park.
So now any ticket can be used for any park.
What they will do in the future if they build theme park #5 is unknown. But since Gail's ticket states "Not valid for parks commencing operations after 5/1/1998", it seems like they will do an upgrade fee for any new park as well. But so far, there have been no theme parks opened after 5/1/1998.
