Can I switch from a calendar year to a fiscal year? Or vice versa?

There is no supported way to change from a calendar year to a fiscal year (or vice versa) while you are still in the year. However, you can make the change for the start of the following year. To do this, you may need to create a (short or long) gap year, or modify your periods creatively to force the accounting periods to sync up with the new year type.