I can't even access my rewards history. Man, how fucked up must Best Buy IT be if it takes like 3 weeks to move a system like Rewards over to a new platform or whatever. I think in just about any modern tech company your entire team would get fired if you said anything would take 3-4 weeks and would be unavailable while its happening.
Working for a major IT company providing services to another major company, I can say while not common, it is also not unusual. The previous system may have been held together with string and bubble gum or built by a third company that no longer exists, and converting the data may be something approaching a manual effort.
They don't lose money except in a nebulous "customer service" line item, so it may have been acceptable to do it during a slower time period (not in late November).
Obviously, they would prefer not to have it down at all, but sometimes you have no choice if you want to move forward.