The big problem with using the 6 hour day to accommodate a rattler is you will be down those bodies for 2 hours once they leave. The popular day shifts before a mid seem to be 0530-1330 or 0600-1400. With either of them, if you just work 0530-1130 or 0600-1200, well from 1130-1330 or 1200-1400, you are down those bodies you used to have today. The larger facilities might be able to make that happen without shifting other things around and just "suffer through it" but the smaller facilities must change something around as they can't lose those bodies for 2 hours. It won't be just plug-and-play with this "new" rattler like some people think.
But straight shifts, you are bang on. It doesn't change any staffing numbers at all. Like you said, instead of 5 different people working a shift for a 5 day period, it'll be the same person instead. No different than if they did a shift swap today for an entire week to work that shift.