In the current "make an announcement, but it's ok to change your mind later about it" environment, here's how that should go down.
plan for new giga coaster (or similar)
determine demolition timeline needed for ToT
order new giga coaster, and ensure financing arrangements are taken care of
Announce new ride, and demolition of TOT same day. Announce closing date of ToT within ~few weeks.
Make big noise about one last ride. Maybe run a competition on a radio station for people to win seats on the last ride ever.
give them some sort of one of a kind or limited edition souvenir from the ride
close the ride
immediately begin demolition
if you've timed it right, you can also begin surveying the site for the new coaster (and you should have been surveying the other non TOT parts of the ride path already
If you've timed it right, ride parts should begin arriving in the carpark.
Once demolition is complete, start construction.
THAT is how you replace a ride. (and i hope Movie World and Sea World are watching this too.