I completely agree that placing a sign that the service is currently under maintenance and that it will be reactivated after several hours would be appropriate with providing access for users who have pending transactions related to how their transactions are executed at that time.
I think the current behavior is understandable if they are making major changes such as renaming the service and stopping the current service.
No matter what kinds of changes you're making to your website, it's better to put up a coming soon or under construction message instead of just showing people some generic error saying the site is unavailable. If someone sees that error out of nowhere, they might get frustrated and not want to come back later. The pro web developers know you gotta keep giving folks a good experience, even when you're working on stuff behind the scenes.