The switch from one system to another shouldn't mean rebooking everyone. One file can carry your whole schedule, past and future, and the import sorts it out.
The cutoff day
Choose Appointments on the import page, then pick a cutoff day in the review step; it starts as today. Rows before it are kept as past visits, with an outcome of attended, no-show or cancelled. Rows on or after it become real, confirmed appointments you can open and chart like any other. Columns are date (a time in the same cell works too), time, status, the patient, service, provider, location and an optional length in minutes.
Matching services, providers and locations
Service, provider and location are matched by name or id. Each name in your file that isn't one of yours yet is listed once above the review table; choose the match and every row using it updates. Rows still waiting on a match are held back while the rest of the file imports. Past-visit rows need none of these. A blank time books at 09:00 with a warning, and a blank length uses the service's length.
A patient who isn't found doesn't fail the row either: tick Add patients that aren't found as new patients and rows with the same first and last name share one new patient.
It accepts what already existed
An import is data from elsewhere, so it doesn't second-guess it. An appointment that overlaps another for the same provider is still imported and shows on the calendar flagged like a staff override. The Google Calendar busy check is skipped. Uploading the same file again skips appointments a patient already has at that time, and a wrong run can be taken back with Undo an import.