Your statement is incorrect it is possible to change or update the date of a Google calendar event using the Google Calendar API.
Option nr 1:
You can patch the event in the body you include the variables you would like to update or patch. In this case date.
Events: patch Updates an event.
Request
PATCH https://www.googleapis.com/calendar/v3/calendars/{calendarId}/events/{eventID}?key={YOUR_API_KEY}
{
"start": {
"dateTime": "2008-09-26T12:01:00+02:00"
}
}
Response
200 OK
Option nr 2
Another option would be to first use Events.get to get the event change what you need then do events.update. This will do an update on everything in the event. While Patch just changes one what you need.