New medicare licensed question

I work in LTC and we have just received out Medicare certification, so my question is if one of our current residents go to the hosp, and come back and meedt the qualifications for Med A, would they just stay on their MDS schedule but would I start the PPS with a 5 day? I figured it wouldn't be the re- admission since they were not a Med A to begin with, also if they come back and are a sig change could I do a sig change and combine it with the 5 day? I am the new medicare nurse and medicare MDS so this is all new to all of us. I figure the majority of our med A will already be current residents that go the hos[ for the 3 day stay. Does the discharging Dr have to write the skilled med A order or if they are sent back without that and I know they can be skilled on Med A can their PCP Dr that covers our residents write that after they return? Any help is appreciated, Thanks