When my day job migrated to Workday, I'm not sure how it was tested ahead of time, but they shut down access to the previous HR system for something like 2 weeks. Which was interesting, as it was the system people used to log hours and stuff and so the hourly folks had to pretty much pre-submit their time for those weeks. You had to make sure any vacation requests and stuff were in for that time and all that.
But that was a few years ago now, and we didn't have a time clock system like Target does, so they may be able to move parts of it and minimize disruptions. The only thing I use eHR for is accessing myTime, so having most systems unavailable are less of an impact to store TMs than it would be to, say, corporate employees.