Is it possible to reassign an existing event from one Solspace Calendar to another ?
This currently isn't built into Calendar, but if your calendars use the same fields, you should be able to manually adjust the calendarID column in the craft_calendar_events database table. Otherwise, you'll have to delete the event and create it again for the correct calendar.

Sometime in the future we plan on seeing if we can add the option to allow changing of calendars. The issue for us is that each calendar is able to have a different field layout and tabs, etc, so it complicates things. :)