It would make the most sense to do a slow transition. That way, as the staff has a slow or down time, they could remove the book out of the card catalog and put it on the computer.
The Chicago Public Library still has a lot of old books that aren’t in the computer and are basically in limbo. If you go to the downtown library and pick out a book before say 1940 and try to check it out, most likely this won’t work. The clerk will say, “Sorry I can’t check this out. You need to go to the reference librarian on the floor where you got it and have her put it in the system. Then come down and I’ll check it out for you.”
Oddly enough they won’t put it in the system till you check it out. I tried this three times, I had books and they weren’t in the system. So I said, “OK I won’t check it out.” I figured the clerk would set it aside and have the librarian put it in the system. But the clerk just throws it in a pile to be reshelved. Unless you specifically ask to have it put in the system, the book will stay in limbo