They've done it form me
I've been a "frequent flyer" (pointless) on Easyjet between my home town and town of residence (both UK) for a number of years. Usually I book the flight I know I can definitely make according to my duties. Often I have turned up two or three hours earlier. In that case I ask the Easyjet enquiries desk (not the checkin - they're too busy) if they can transfer me to the currently loading flight. This has been done without extra charge two out of three times.
However, I never tried this overnight.