Is not uncommon that very critical databases remain on obsolete versions longer than it is convenient.
If the database provide good service and reasonable performance, the risks related to any radical change make the people that need to make the decision to upgrade, to delay it as much as possible.
Sooner or later the day come when the increasing amount of problems, usually related to a big increment on the load and complexity of the applications running on the database raise up the need to upgrade to a more powerful version.
At this moment the DBA get a small window to shutdown the application during the weekend and upgrade the database as fast and safely as possible.
The attached document: "220.127.116.11 - 32 bits Database Upgrade to 18.104.22.168 - 64 bits" is the checklist I did prepare for such a case.